MailTestCase changed to inherit from SoCTestCase instead of TestCase.
MailTestCase changed to inherit from SoCTestCase instead of TestCase.

Therefore, it will have access to its valuable members, like testbed.
Also, run_tasks function moved inside MailTestCase class. By giving it access to MailTestCase instance, the function will use its testbed and test Django client. Duplicated code to support initialization is not needed.

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