chore: update systest creds and fix unit test syntax (#1210)

* chore: update systest creds

* chore: fix unit test

Author: arithmetic1728

packages/google-auth/system_tests/secrets.tar.enc

@@ -373,7 +373,7 @@ def test_refresh_with_jwt_credentials(self, make_jwt):
         assert credentials.valid
 
         # Assert make_jwt was called
-        assert make_jwt.called_once()
+        assert make_jwt.call_count == 1
 
         assert credentials.token == token
         assert credentials.expiry == expiry