Add PHPUnit tests

This test includes all main scenarios for ReminderService:
Untranslatable entity → no warning
No translations → no warning
Disabled bundle → no warning
Enabled bundle with custom message → warning shown
Enabled bundle with default message → warning shown

Test look fine but there should be a proper documentation for this test class and methods which is currently missing.

Pending: docBlock for class and methods.

Thank you so much @miroslavstankov

Happy to merge this once the PHPCS and PHPSTAN errors are fixed :-). Assigning credits.

In case you don't know, you can see the reports in the GitLab runner results, and some are visible on the merge request diff.

I'll look into the PHPCS / PHSTAN errors

Fixed the PHPCS and PHPSTAN errors, kindly review.