tests/helper-runtests.sh

tests: use readlink.py instead of readlink

The latter doesn't always exist, such as on Solaris-derived platforms.

Differential Revision: https://phab.mercurial-scm.org/D996

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL