Add test registry server

[somtochiama]

Source-controller and pkg/oci both set up a registry server for tests. It would be great to have a package for this so it doesn't need to be set up in each repository. The new pkg should also discard verbose logging as this can be distracting in tests as noted in @darkowlzz comment https://github.com/fluxcd/pkg/pull/328#issuecomment-1230738920

The image-automation controller also uses a different test registry. It should be updated so the pkg is used everywhere