Add xmldb checks

[ewallah]

MDL-73764 added a Behat script to reconcile install.xml files.

In Github Actions, one can use this check to see if a plugin has differences between the current install.xml and what the XMLDB editor saves:

      - name: xmldb
        if: ${{ always() }}
        run: php admin/tool/behat/cli/run.php --tags=@tool_xmldb
        working-directory: moodle

Of course, a PHPUnit script would do the job faster and provide better error output.