Fix the RepositoryForm unit tests, and make them less error-prone.

My final work for the Bitbucket support broke RepositoryForm unit tests,
and I apparently failed to test those. So, this change fixes it, by
making use of a TestService instead of using an existing hosting service
for tests. This way, we can control things like authorization (which is
what broke the tests before), repository checking, fields, etc.

All unit tests pass.