This documents our existing testing best practices, and tries to explain
some of the rationale around them.

There’s some new policy around performance tests, specifying that they
are good to keep around if they are high quality (so they can be used
again, the effort put into them not thrown away, and the numbers
compared over time) and that they should be runnable automatically to
avoid bitrot.

Everything else just documents existing practice.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>