Add performance testing

[dcermak]

@dirkmueller anything in particular that you'd like to check?

[evrardjp]

I have started a PR with that, for which we have a new tox unit (therefore we have a single isolated test). That tox unit will not fetch the containers in advance, and should build everything in one place. The test timings from pytest (through junit output) is a good start, but if we need more details, we should maybe add a pytest plugin.