Test both Rugged and Gitaly paths

In the test environment spec/support/gitaly.rb enforces that Gitaly will always be used, and in the development environment, GitalyClient#migrate enforces the same.

However, we see bugs such as gitlab-org/gitlab-ce#41735 because the Rugged paths are no longer tested automatically.

What's the best way we can automate testing of both until Rugged is gone?