gitlab-sshd: acceptance tests

The idea here is to stand up a gitlab-sshd next to a fake rails and a gitaly container, and run each command through an SSH client.

We can start with simple success tests for each command. Here's the list:

The following require us to stand up a Gitaly server to test properly, as we do for the gitlab-elasticsearch-indexer CI and tests, so will need further framework code:

Edited by Vasilii Iakliushin