run-tests: allow run-tests.py to run tests outside current directory
When reloading tests, run-tests.py was assuming that it could look
up the test by the basename, which only works if you are running
tests which are in the current directory.
This patch changes that lookup to use the full path. This is all
that was needed, and does not appear to cause any problems for
any of the existing testing work flows based on running the
suggested commands at the top of run-tests.py.
Motivation: In order to test Mercurial with Hypothesis (according
to https://www.mercurial-scm.org/wiki/HypothesisPlan) it is
useful to be able to generate temporary test files and execute
them. Generating temporary files in the tests/ directory leads to
a lot of suboptimal clutter.