Fixed Unit test for magnum-ui

After 7052b7f065 merged in openstack/horizon a new Unit
test class introduced in horizon i.e. RestAPITestCase.
This patch updates magnum-ui to import RestAPITestCase
class instead of TestCase. For more info. see [1]

[1] https://review.opendev.org/c/openstack/horizon/+/827097/2/openstack_dashboard/test/helpers.py#485

Change-Id: I4ef0cd315e615e390f15a8fa16fe4aaa380a2470

magnum_ui/test/api_tests/test_rest_api.py

@@ -24,7 +24,7 @@ from openstack_dashboard.test.test_data import utils
 TEST = utils.TestData(test_data.data)
 
 
-class MagnumRestTestCase(test.TestCase):
+class MagnumRestTestCase(test.RestAPITestCase):
 
     # Cluster Templates
     @mock.patch.object(magnum, 'magnum')