Fixed broken RescueGroupsController test

* Because current_user is called upon each call to the controller,
  in the test we have to call a before(:each) which calls the stubbed
  method before each example. You can't call it in a before(:all) because
  the particular stubbed method (:current_user) is called every example.
  So this *must* be a before(:each) rather than a before(:all) as a result.