Document Chutney in our test suite doc

Parent task: #15864 (closed)

@boyska was confused while running our test suite: while Chutney was still bootstrapping, he expected the TailsToaster VM to be started already, and thought something was not working properly because there was no QEMU process running yet.

It would be good if https://tails.boum.org/contribute/release_process/test/automated_tests/ mentioned Chutney briefly, at least to tell what it is and what we're using it for.