Document new test-suite backend

[dmnks]

Add a bit more text to tests/README.md about the new test-suite layout from #2559 and while at it, write a short howto on test writing, too. A file describing the mktree interface (mktree.README) may also be helpful. Lastly, a Known issues section wouldn't hurt either since we have at least two known, intermittent false positive failures, as mentioned in #2470.

[dmnks]

For now, let's just focus on the user side of things in the README.md file. The internal architecture of the test suite will be described later, perhaps together with a "how to write a test" section.

[pmatilai]

As discussed separately, dropping the milestone afterall: this is basically developer-only docs, and developers are expected to be looking at git master, not release tarball. So it's not critical that this documentation makes it to rc.

[dmnks]

Yup, thanks for bringing us back to sanity here 😄 It really can wait, more so that we're still planning some changes to the test suite implementation.