Fix: Expected test values

* For the BucketsControllerTest#addCollaborator and
RiversControllerTest#addCollaborator do not check for the exact
value of the returned collaborator_id. This is because JUnit doesn't guarantee
the test execution order so a if test case that adds an either a bucket or river
collaborator is executed before the controller test, the returned id will be
different as the RDMS rollback does not affect the AUTO_INCREMENT value

* Remove unnecessary imports