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/php-compatibility.yml

@@ -49,6 +49,7 @@ jobs:
   # - Make Composer packages available globally.
   # - Logs PHP_CodeSniffer debug information.
   # - Runs the PHP compatibility tests.
+  # - Ensures version-controlled files are not modified or deleted.
   # - todo: Configure Slack notifications for failing scans.
   php-comatibility:
     name: Check PHP compatibility
@@ -84,3 +85,6 @@ jobs:
 
       - name: Run PHP compatibility tests
         run: phpcs --standard=phpcompat.xml.dist -q --report=checkstyle | cs2pr
+
+      - name: Ensure version-controlled files are not modified or deleted
+        run: git diff --exit-code