tests/helper-runtests.sh

chgserver: update the umask cache before each run

posix.py uses a global variable to store the umask value resulting in caching of
it when using chg. We need to update it before each command run as the umask can
change between commands.

This fixes test-inherit-mode.t with chg.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL