By relocating helper methods into a mixin class, we can be more flexible
with managing transactions in test cases, without always forcing the
django.test.TestCase behavior of always putting the test case into an
atomic transaction.
We include a check for side effects in ZulipTransactionTestCase. It only
checks for the set of row ids in all tables before and after each test.
It is not a comprehensive check for side effects, but should be
sufficient for the basics without much performance overhead.
Zixuan James Li