docs: Add a testing policy

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