Gitaly comparison tests
Closes #35 (closed).
This is a new test harness for HGitaly, complete with a fixture based on the GitLab mirroring capability of py-heptapod. It is complemnetary to the existing integration tests and isn't supposed to replace them.
It assumes that HGitaly is part of a HDK workspace or preinstalled in CI. At this point, this is all that we need, but it can be improved later on if needed.
The CI setup runs the Gitaly tests only for the "current" Mercurial version, as can be found
heptapod-base CI image. We won't have appropriate base images for other versions.
Coverage policy stays the same. The Gitaly comparison tests themselves are skipped if
a Gitaly installation is not available, for example in the
compat pipeline stage.
The README grows a big section about the different kinds of tests and the testing policy. It comes with a hints subsection, to make it nicer than rules, rules and more rules.