* Added GraphQLTransactionTestCase
- Adds support for testing code that is executed within a transaction
Reference: https://docs.djangoproject.com/en/3.1/topics/testing/tools/#django.test.TransactionTestCase
```
For instance, you cannot test that a block of code is executing within a transaction, as is required when using select_for_update(). In those cases, you should use TransactionTestCase.
```
* Update testing.py
* Update testing.py
* Fixed formatting.
* Updated docs.
* Updated test.
* Update testing.rst
* Use the Django Client test utility instance that Django provides with its TestCase class. This allows GraphQL tests to make use of the stateful client methods like login()
* Add missing test case initializer call
* Don't break backward compability
* Add test for pending deprecation warning on GraphQLTestCase._client
Co-authored-by: Tom Nightingale <tom@tnightingale.com>
