tests/helper-runtests.sh

deb: avoid use of [[ in 'rules' file

It's not supported by posix shell, and apparently my build system uses that.

Differential Revision: https://phab.mercurial-scm.org/D10292

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL