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]
