run-tests: disable color on Windows
More Windows sadness. Maybe someone can figure out how to make win32 color
work, but I think we avoid importing stuff from the mercurial package in this
module. On the plus side, this conditionalizes away a test failure.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL