Write test for PATCHing submission, confirming you cannot change Phase of submission

https://github.com/codalab/competitions-v2/blob/5ef96c2048f9f8750f12eef677c23c5dbab1ff43/src/apps/api/serializers/submissions.py#L135

Resolve this TODO, write a test confirming PATCH for submissions doesn't allow changing the phase the submission belongs to.