Better transaction management in Django TestCase.

Atomiq publish will throw an exception if not used within a transaction, and we want to make sure nobody ships code that is going to throw this exception in production. The problem is that Django TestCase wraps all test functions in a transaction, which masks this error.

This release makes a few changes to the transaction checking:

1. If we are in a TestCase unit test function, we expect to find 3 transactions total: two created by Django and one by the user.

2. If we are in the TestCase setUpClass function, we expect to find 2 transactions: one created by Django and by the user.