Yes, I can add a test checking that the seeds generated are the same both for ground_motion_fields=True and ground_motion_fields=False. At the moment there are no tests checking the case ground_motion_fields=False, this is why the change did not affect any test.
A test is missing, but I am going to add it very soon, see https://bugs.launchpad.net/oq-engine/+bug/1282894