Add a TestCase mixin for enabling caches in tests

By default, disable all caching in tests, to preserve test independence.
In order to enable caching, inherit from CacheSetupMixin, and specify
which cache configuration is needed.

[EV-32]

lms/djangoapps/bulk_email/tests/test_err_handling.py

@@ -44,6 +44,8 @@ class TestEmailErrors(ModuleStoreTestCase):
     Test that errors from sending email are handled properly.
     """
 
+    ENABLED_CACHES = ['default', 'mongo_metadata_inheritance', 'loc_cache']
+
     def setUp(self):
         super(TestEmailErrors, self).setUp()
         course_title = u"ẗëṡẗ title ｲ乇丂ｲ ﾶ乇丂丂ﾑg乇 ｷo尺 ﾑﾚﾚ тэѕт мэѕѕаБэ"