Unit tests improvements

A series of unit tests improvements to make:

- [ ] Add tests for `SimperiumTagsRepository`
- [ ] Use `Mockito` runner for the unit tests
- [ ] Remove warnings in bucket mocks (see `SimperiumCollaboratorsRepositoryTest`)
- [ ] Add proper tests for `collaboratorsChanged()` in `SimperiumCollaboratorsRepository`

Locate the unit tests for SimperiumTagsRepository and SimperiumCollaboratorsRepository, including SimperiumCollaboratorsRepositoryTest, and read how bucket mocks are currently configured. Work through the checklist: add the missing repository and collaboratorsChanged() coverage, use the Mockito runner, and remove mock warnings; done means each listed test improvement is covered without warnings.

Written by the indexing model from the issue text.