tests/helper-runtests.sh

test-issue660: add dirstate-v2 variant

It's basically a dirstate test, so it makes sense to test out the new version.

Differential Revision: https://phab.mercurial-scm.org/D12445

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL