watch: add --no-clean to nyc args on change tests

Not sure how this went so long without me noticing.  Probably because I
usually use a coverage-map, so one test = 100% of one file.

Author: isaacs

lib/watch.js

@@ -112,7 +112,7 @@ class Watch extends Minipass {
     this.queue.length = 0
 
     this.proc = spawn(process.execPath, [
-      ...this.args, '--save=' + this.saveFile
+      ...this.args, '--save=' + this.saveFile, '--nyc-arg=--no-clean'
     ], {
       stdio: 'inherit',
       env: this.env,