Compliance test report should contain info about test suite version

[mbuechse]

To this end, introduce a double-underscore version variable.

This is particularly relevant for those CSPs that run the tests themselves.

Originally, this proposed to include the commit id, but it might be possible that no "actual" git checkout is used -- as is the case with Zuul (which uses some kind of mock checkout), or with pip (assuming that we make the test suite installable via pip).

Originally, this also speculated that git status could be used to detect any form of hampering, which doesn't work for the same reason. We can compute some kind of hash of the most important files though. I would leave that for a future issue. (YAGNI)