tests/helper-runtests.sh

packaging: extract py2exe functionality to own module

py2exe builds are shared between Inno Setup and WIX. We'll
want the logic for performing py2exe builds to be reusable
across the code for both installers.

This commit extracts the py2exe-specific functionality into
its own module.

There's definitely room to customize things further. This will
be done in future commits, as necessary. (I'm not even sure what
customizations WIX will require yet. Presumably a lot.)

Differential Revision: https://phab.mercurial-scm.org/D6091

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL