[PATCH] run-tests: allow run-tests.py to run tests outside current directory

[timeless]

David MacIver wrote:
> The comment at the top of run-tests.py suggested that testing of it was
> manual, which is why I didn't think to look for similar tests. Would you be
> amenable to a second patch to amend the comment?

tests/test-run-tests.t exists, and could be listed as a 0 step, but,
it's not that simple.

Basically test-run-tests.t can handle basic features, but a change to
run-tests.py can easily be only found in a certain mode and a certain
bigger test. Sometimes it's possible to enhance test-run-tests.t,
sometimes it's /probably/ too hard.

I haven't figured out how to write a useful comment, but I'd be
willing to offer feedback on a suggestion.