Adding more tests around `auth.ResolveAuthConfig`

mostly because I've been failing at getting docker `0.6.2` working with docker-registry `0.6.0` with regard to login and push.  I suspect there may be some mismatched expectations between `auth.ResolveAuthConfig` and `registry.ResolveRepositoryName`, but I haven't done enough research or experimentation to think it's anything more than user error.  More tests are good, no?