Remove hardcoded test repo names

While we work on removing the manual effort of test repo creation we can make it easier to test right now by removing the hardcoded repository names without dependency on those changes.

I added a global `@test_repo` variable that returns a full repo name built from `test_github_login/test_github_repository`.

Additionally with these changes I will be adding the setup and teardown needed to rerecord the cassette without changing the test or further manual effort. In 0eb279f I added a call to get the latest commit sha to add a status, for example.