test: install WebdriverIO packages as optional dev dependencies (#510)

WebdriverIO which we use for browser testing cannot be installed on some
platforms like z/OS.

In order to allow "Canary in the Goldmine"
(https://github.com/nodejs/citgm) to be run on such platforms as well we
need to install WebdriverIO as an optional devDependency. npm doesn't
support this out-of-the-box but fortunately there is
https://github.com/bcoe/optional-dev-dependency which does precisely
what we need.

Fixes #509

examples/browser-rollup/package.json

@@ -10,8 +10,8 @@
     "uuid": "file:../../.local"
   },
   "devDependencies": {
-    "@rollup/plugin-node-resolve": "^8.0.1",
-    "rollup": "^2.18.0",
-    "rollup-plugin-terser": "^6.1.0"
+    "@rollup/plugin-node-resolve": "^9.0.0",
+    "rollup": "^2.25.0",
+    "rollup-plugin-terser": "^7.0.0"
   }
 }