Better support specifying of modulestore configuration in test cases

The existing pattern of using `override_settings(MODULESTORE=...)` prevented
us from having more than one layer of subclassing in modulestore tests.

In a structure like:

    @override_settings(MODULESTORE=store_a)
    class BaseTestCase(ModuleStoreTestCase):
        def setUp(self):
            # use store

    @override_settings(MODULESTORE=store_b)
    class ChildTestCase(BaseTestCase):
        def setUp(self):
            # use store

In this case, the store actions performed in `BaseTestCase` on behalf of
`ChildTestCase` would still use `store_a`, even though the `ChildTestCase`
had specified to use `store_b`. This is because the `override_settings`
decorator would be the innermost wrapper around the `BaseTestCase.setUp` method,
no matter what `ChildTestCase` does.

To remedy this, we move the call to `override_settings` into the
`ModuleStoreTestCase.setUp` method, and use a cleanup to remove the override.
Subclasses can just defined the `MODULESTORE` class attribute to specify which
modulestore to use _for the entire `setUp` chain_.

[PLAT-419]

common/djangoapps/external_auth/tests/test_shib.py

@@ -73,7 +73,7 @@ def gen_all_identities():
 
 
 @ddt
-@override_settings(MODULESTORE=TEST_DATA_MOCK_MODULESTORE, SESSION_ENGINE='django.contrib.sessions.backends.cache')
+@override_settings(SESSION_ENGINE='django.contrib.sessions.backends.cache')
 class ShibSPTest(ModuleStoreTestCase):
     """
     Tests for the Shibboleth SP, which communicates via request.META