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?
Merge pull request #1990 from modcloth-labs/more-tests-for-auth-registry-resolution

Adding more tests around `auth.ResolveAuthConfig`