TestCase _fixture_teardown breaks database MIRROR usage

[Jean-Daniel]

The TestCase class _fixture_teardown() implementation try to iterate over all databases while it should ignore MIRROR databases.

Trying to include MIRROR database in the iteration loop make the teardown be execute twice on the database and raise an exception.

  for db_name in reversed(self._databases_names()):

Should be

  for db_name in reversed(self._databases_names(include_mirrors=False)):

[Mariusz Felisiak]

I think it's a duplicate of #23718. TEST['MIRROR'] requires TransactionTestCase to work with multiple databases.