A while back we had made changes to the hosting services to require keyword arguments for `check_repository`. In the Assembla implementation, this change ended up changing the `assembla_project_id` field to not include a default value. This field is required when using a Perforce repository, but is not when using SVN. In practice, the repository form will always provide this, but we had a unit test that didn't. To prefer existing compatibility, I've given this field a `None` default again, and raise a `ValueError` if it's not provided in the Perforce case. While in here I also noticed that the skip messages for unit tests with SVN repositories were wrong. Testing Done: Ran unit tests.