test: re-enable migration checks (#36655)

the check was disabled:
openedx/edx-enterprise#2338

because we needed to fake a migration:
openedx/edx-enterprise#2383

but that fake migration is now in edx-platform:
openedx/edx-enterprise#2383

so we can re-enable the check.

Author: jesperhodge

common/djangoapps/util/tests/test_db.py

@@ -1,7 +1,6 @@
 """Tests for util.db module."""
 
 from io import StringIO
-import unittest
 
 import ddt
 from django.core.management import call_command
@@ -121,7 +120,6 @@ class MigrationTests(TestCase):
     Tests for migrations.
     """
 
-    @unittest.skip('Skipping temporarily to add a foreign key')
     @override_settings(MIGRATION_MODULES={})
     def test_migrations_are_in_sync(self):
         """