tests/helper-runtests.sh

test-pathconflicts-merge: stop requiring symlink support

The errors from the last time I took a shot at this back in early November have
disappeared, so let's just enable this.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL