If PHPUnit is not installed, ignore `tests/src` directories

[mglaman]

If PHPUnit isn't added to the Drupal project, let's add tests/src as excluded directories to avoid TestCase class not found errors

[mglaman]

Alternatively, adding phpstan/phpstan-phpunit as a dependency would provide the stubs and might be an easier work around.

[mglaman]

Maybe we'll keep this.