run-tests: make coverage work out of tree
Currently coverage fails when run on an out-of-tree extension since
run-tests.py tries to load sitecustomize.py from self._testdir, which is the
dir for the extension's tests.
Differential Revision: https://phab.mercurial-scm.org/D6960
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed