How to use the generator-jhipster/generators/generator-constants.SERVER_TEST_SRC_DIR function in generator-jhipster

if (this.jhipsterAppConfig.buildTool === 'maven' && (this.jhipsterAppConfig.clientFramework === 'angularX' || this.jhipsterAppConfig.clientFramework === 'angular2')) {
            var fs = require('fs');
            // read config from .yo-rc.json
            this.baseName = this.jhipsterAppConfig.baseName;
            this.packageName = this.jhipsterAppConfig.packageName;
            this.packageFolder = this.jhipsterAppConfig.packageFolder;
            this.clientFramework = this.jhipsterAppConfig.clientFramework;
            this.clientPackageManager = this.jhipsterAppConfig.clientPackageManager;
            this.buildTool = this.jhipsterAppConfig.buildTool;

            // use function in generator-base.js from generator-jhipster
            this.angularAppName = this.getAngularAppName();

            // use constants from generator-constants.js
            const javaDir = `${jhipsterConstants.SERVER_MAIN_SRC_DIR + this.packageFolder}/`;
            const javaTestDir = `${jhipsterConstants.SERVER_TEST_SRC_DIR + this.packageFolder}/`;
            const resourceDir = jhipsterConstants.SERVER_MAIN_RES_DIR;
            const webappDir = jhipsterConstants.CLIENT_MAIN_SRC_DIR;

            // variable from questions
            this.pubStripeKey = this.props.pubStripeKey;
            this.priStripeKey = this.props.priStripeKey
            //******************************************
            //
            // Adding a new payment entity
            //
            //******************************************

            var shell = require('shelljs');
            //adding ngx-stripe dependency
            if (shell.exec('npm install ngx-stripe').code !== 0) {
                shell.echo('Error: ngx-stripe installation failed');

writeFiles: function () {
                if (!this.grpcService) {
                    return;
                }
                // function to use directly template
                this.template = function (source, destination, context) {
                    this.fs.copyTpl(
                        this.templatePath(source),
                        this.destinationPath(destination),
                        this,
                        context
                    );
                };
                const grpcEntityDir = jhipsterConstants.SERVER_MAIN_SRC_DIR + this.packageFolder + '/grpc/entity/' + this.entityUnderscoredName ;
                const grpcEntityTestDir = jhipsterConstants.SERVER_TEST_SRC_DIR + this.packageFolder + '/grpc/entity/' + this.entityUnderscoredName ;
                this.log(grpcEntityDir);
                this.template('_entity.proto', 'src/main/proto/' + this.packageFolder + '/entity/' + this.entityUnderscoredName + '.proto');
                this.template('_entityProtoMapper.java', grpcEntityDir + '/'+ this.entityClass + 'ProtoMapper.java');
                this.template('_entityGrpcService.java', grpcEntityDir + '/'+ this.entityClass + 'GrpcService.java');
                this.template('_entityGrpcServiceIntTest.java', grpcEntityTestDir + '/'+ this.entityClass + 'GrpcServiceIntTest.java', { context: { randexp, _, chalkRed: chalk.red, fs, SERVER_TEST_SRC_DIR: grpcEntityTestDir } });
            },

* Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
const mkdirp = require('mkdirp');
const cleanup = require('generator-jhipster/generators/cleanup');
const constants = require('generator-jhipster/generators/generator-constants');
const baseServerFiles = require('generator-jhipster/generators/server/files').serverFiles;

/* Constants use throughout */
const INTERPOLATE_REGEX = constants.INTERPOLATE_REGEX;
const SERVER_MAIN_SRC_DIR = constants.SERVER_MAIN_SRC_DIR;
const SERVER_MAIN_RES_DIR = constants.SERVER_MAIN_RES_DIR;
const SERVER_TEST_SRC_DIR = constants.SERVER_TEST_SRC_DIR;
const SERVER_TEST_RES_DIR = constants.SERVER_TEST_RES_DIR;

/* TODO: Do a PR in the parent JHipster project to export and re-use here as well in order to have a single source of truth!!!
const TEST_DIR = constants.TEST_DIR;
const shouldSkipUserManagement = generator =>
    generator.skipUserManagement && (generator.applicationType !== 'monolith' || generator.authenticationType !== 'oauth2');
*/

/**
 * The default is to use a file path string. It implies use of the template method.
 * For any other config an object { file:.., method:.., template:.. } can be used
 */
const serverFiles = {
    ...baseServerFiles,
    serverResource: [
        {

this.packageName = this.jhipsterAppConfig.packageName;
                this.applicationType = this.jhipsterAppConfig.applicationType;
                this.authenticationType = this.jhipsterAppConfig.authenticationType;
                this.databaseType = this.jhipsterAppConfig.databaseType;
                this.searchEngine = this.jhipsterAppConfig.searchEngine;
                this.skipUserManagement = this.jhipsterAppConfig.skipUserManagement;
                if (this.applicationType === 'gateway' && this.authenticationType === 'uaa') {
                    this.skipUserManagement = true;
                }
                this.buildTool = this.jhipsterAppConfig.buildTool;
                this.cacheManagerIsAvailable = ['ehcache', 'caffeine', 'hazelcast', 'infinispan', 'memcached', 'redis'].includes(this.jhipsterAppConfig.cacheProvider) || this.applicationType === 'gateway';
                this.cacheProvider = this.jhipsterAppConfig.cacheProvider || this.jhipsterAppConfig.hibernateCache || 'no';
                this.messageBroker = this.jhipsterAppConfig.messageBroker;

                const javaDir = `${jhipsterConstants.SERVER_MAIN_SRC_DIR + this.packageFolder}/`;
                const testDir = `${jhipsterConstants.SERVER_TEST_SRC_DIR + this.packageFolder}/`;
                const protoDir = jhipsterConstants.MAIN_DIR + 'proto/';
                const protoPackageDir = protoDir + this.packageFolder + '/';

                if (this.databaseType === 'sql' && this.authenticationType !== 'oauth2') {
                    this.idProtoType = 'int64';
                    this.idProtoWrappedType = 'Int64Value';
                } else {
                    this.idProtoType = 'string';
                    this.idProtoWrappedType = 'StringValue';
                }

                this.template('_date.proto', protoDir + 'util/date.proto');
                this.template('_decimal.proto', protoDir + 'util/decimal.proto');
                this.template('_pagination.proto', protoDir + 'util/pagination.proto');
                this.template('_queryfilters.proto', protoDir + 'util/queryfilters.proto');