tests/helper-runtests.sh

test-rebase-interruptions: stabilize for Windows

External hooks end up launching cmd.exe, which knows nothing about $VAR syntax.
For some reason, I thought that Mercurial would substitute in the value, in
order to paper over the platform difference.  But I can't find that in the
documentation, and there's at least one other use of this pattern [1].

[1] https://www.mercurial-scm.org/repo/hg/file/tip/tests/test-histedit-fold.t#l477

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL