notification tests: delete notifications before (not after) tests

Don't clean notifications (and changeset comments) *after* the test,
but *before* the test. Other unit tests don't care if they leave
notifications in the database, and neither should these. Rather,
they should ensure their *own* preconditions before testing.

Admittedly, currently only one test leaves a notification in the
database, but more could come along at any time (and why worry?):
TestPullrequestsController.test_create_with_existing_reviewer