Issue #3556293: Fix hardcoded module path in PhpLintTest
The PhpLintTest class had a hardcoded path 'modules/custom/environment_indicator' which caused test failures when the module was installed in a different location (e.g., modules/contrib/ via Composer).
This fix replaces the hardcoded path with dynamic path resolution that:
- Uses dirname(DIR, 3) to navigate from the test file to the module root
- Converts the absolute path to a relative path from DRUPAL_ROOT
- Works correctly regardless of installation location (contrib, custom, etc.)
The test now passes regardless of where the module is installed.
Closes #3556293