tests/helper-runtests.sh

fsmonitor: access repo.root

There is no repo._root. It looks like fsmonitor has
been busted since this access was introduced in
ab1900323b1 in July 2019!

Differential Revision: https://phab.mercurial-scm.org/D7207

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL