I'd move these new tests under a new class named `class SendPendingMembershipEmailsTests`. Generally the pattern we should follow is this:
 * filename is `test_<base filename of the test containing the code we test>.py`
 * for each `class` or `def` we define a new class named after the later and ending with `Tests`. Something like `ShowTests` for `show` view.

Based on these, `test_membership.py` code should be inside test_models. Since this file was not created in this pull request feel free to leave as is and I can refactor afterwards.

Also in `test_tasks.py` the already existing `TaskTests` class can be renamed to `RemoveEmptyGroupsTests` to follow this naming pattern.