Fixed #21281 -- Made override_settings act at class level when used as a TestCase decorator
Fixed #20392 -- Wrapped TestCase into a class-wide transaction
Refactored queries tests following #20392
Refactored select_related tests following #20392
Turn some TestCase into TransactionTestCase following #20392
Tests that need to close the connection are not compatible with the concept of a global transaction wrapping the whole TestCase