Added multi_db=True to test cases that access the 'other' db connection.

Fixed a failure in the context processors tests when running in
reverse on MySQL due to an extra query after refs #27683.

tests/migrations/test_base.py

@@ -18,6 +18,7 @@ class MigrationTestBase(TransactionTestCase):
     """
 
     available_apps = ["migrations"]
+    multi_db = True
 
     def tearDown(self):
         # Reset applied-migrations state.