tests/helper-runtests.sh

journal: take wlock for writting the 'shared' file

As we did for the shared extension itself, we add some locking around the write
of the 'shared' file.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL