Using a global test repo solves the hardcoded test repo names problem but adds the annoyance of having to worry about deleting the additional setup and teardown cassettes it will use. This is more useless knowledge that will have to be documented and read by anyone wanting to contribute and I sense may cause headaches.

By using individual test repos everything is encapsulated in its own single cassette file, completely isolated to the point that it just feels right and that that is how it is supposed to be. There is a little more setup and teardown code but overall it makes things easier for making changes.