refactor: split 'with_quota_project' into separate base class (#561)

Co-authored-by: Tres Seaver <tseaver@palladion.com>
diff --git a/tests/test__default.py b/tests/test__default.py
index 55a14c2..2738e22 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -49,7 +49,7 @@
 with open(SERVICE_ACCOUNT_FILE) as fh:
     SERVICE_ACCOUNT_FILE_DATA = json.load(fh)
 
-MOCK_CREDENTIALS = mock.Mock(spec=credentials.Credentials)
+MOCK_CREDENTIALS = mock.Mock(spec=credentials.CredentialsWithQuotaProject)
 MOCK_CREDENTIALS.with_quota_project.return_value = MOCK_CREDENTIALS
 
 LOAD_FILE_PATCH = mock.patch(