tests/helper-runtests.sh

sidedata: test we can successfully write sidedata

For now we just write them and rejoice on the lack of crashes.

Differential Revision: https://phab.mercurial-scm.org/D6896

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL