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/phpunit-tests.yml

@@ -55,6 +55,7 @@ jobs:
   # - Logs debug information about what's installed within the WordPress Docker containers.
   # - Install WordPress within the Docker container.
   # - Run the PHPUnit tests.
+  # - Ensures version-controlled files are not modified or deleted.
   # - Checks out the WordPress Test reporter repository.
   # - Reconnect the directory to the Git repository.
   # - Submit the test results to the WordPress.org host test results.
@@ -221,6 +222,9 @@ jobs:
         if: ${{ ! matrix.split_slow }}
         run: LOCAL_PHP_XDEBUG=true npm run test:${{ env.PHPUNIT_SCRIPT }} -- -v --group xdebug --exclude-group __fakegroup__
 
+      - name: Ensure version-controlled files are not modified or deleted
+        run: git diff --exit-code
+
       - name: Checkout the WordPress Test Reporter
         if: ${{ github.repository == 'WordPress/wordpress-develop' && github.ref == 'refs/heads/master' && matrix.report }}
         uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f # v2.3.4