Comparison tests: helpers for the `do_rpc` pattern

In recent comparison tests, we often use this pattern:

• store stubs in a dict with VCS as keys (hg and git)
• provide a do_rpc nested function that takes a vcs parameter and calls HGitaly or Gitaly according to it, using the stubs dict, perhaps performing conversions (changeset SHAs) in request or response.

This allows us to conclude many tests with something like assert do_rpc('hg', ...) == do_rpc('git', ...)

It would be great if we had a helper to do this more readily, as it would help to really focus on the current test case, but care must be taken not to make it an additional layer of obscurity.