Extend module automated tests

TODO:

1. Update unit tests of the ViewsBulkOperationsBatch class.
2. Provide basic functional test for the Views Bulk Operations views row plugin.
   • Add unit tests for module API: altering action definitions, providing contrib views data - postponed, those are simple event subscribers.
   • Add a functional test case that will include use of the Drupal\views_bulk_operations\Controller\ViewsBulkOperationsController class: executing an action with batching but without configuration and confirmation steps.
   • Basic test of action processing.
   • Entity translation check.
   • Configuration and confirmation form tests.
   • Batching test.
   • Tests of "pass view" and "pass context" functionalities.
3. Replace deprecated BrowserTestBase assertions with relevant WebAssert methods.
4. Create unit tests of the views_bulk_operations.processor service class. (obsolete - covered by functional tests)