Wrapped tests now make use of Django's atomic transactions

[zenoamaro]

This patch updates ColorDjangoTestSuiteRunner.wrap_tests in order to use
of Django's atomic transactions, as well as making them available to the
TestCase itself.

These are the parts that needed to be backported from Django:

    # django/test/testcases.py:848
    # https://github.com/django/django/blob/master/django/test/testcases.py#L848
    def _fixture_setup(self):
        # ...
        self.atomics = {}
        for db_name in self._databases_names():
            self.atomics[db_name] = transaction.atomic(using=db_name)
            self.atomics[db_name].__enter__()
        # ...

    # django/test/testcases.py:877
    # https://github.com/django/django/blob/master/django/test/testcases.py#L877
    def _fixture_teardown(self):
        # ...
        for db_name in reversed(self._databases_names()):
            # ...
            self.atomics[db_name].__exit__(None, None, None)