tests/helper-runtests.sh

githelp: format with %d if an integer

Python 3 doesn't allow us to format an int with %s like Python 2
did. So handle that.

Differential Revision: https://phab.mercurial-scm.org/D5726

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL