Konnect entities: add tests checking removal of KongConsumer credentials on KongConsumer deletion

[pmalek]

Problem statement

#709 removed tests checking that KongConsumer removal deletes bound credentials.

This was incorrect to add this test in envtest suite as there is no controller that would assign the deletion timestamp to children (which have owner reference set).

This issue tracks the effort of adding those tests in a different suite (e.g. integration) where a fully functional cluster is running and deletion timestamps are added to entities when their owner is deleted.