Build/Test Tools: Add assertions to ensure version-controlled files are not modified during CI, and fix the grunt clean command.

Some tests and some commands that run during the build steps modify files that are under version control. This adds assertions to ensure that these files don't remain in a modified state after the test runs on CI.

This also fixes the `grunt clean` command which erroneously deletes `script-loader-packages.php`. This file is re-populated during the build, but deleting it during the clean is undesirable.

Fixes #53606


git-svn-id: https://develop.svn.wordpress.org/trunk@51355 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/javascript-tests.yml

@@ -49,6 +49,7 @@ jobs:
   # - Logs updated debug information.
   # _ Installs NPM dependencies using install-changed to hash the `package.json` file.
   # - Run the WordPress QUnit tests.
+  # - Ensures version-controlled files are not modified or deleted.
   # - todo: Configure Slack notifications for failing tests.
   test-js:
     name: QUnit Tests
@@ -82,3 +83,6 @@ jobs:
 
       - name: Run QUnit tests
         run: npm run grunt qunit:compiled
+
+      - name: Ensure version-controlled files are not modified or deleted
+        run: git diff --exit-code