tox: get tests passing a bit on Windows

We need to use the path separators provided by the python library,
and we need to set the git env vars so the name is always known.
Not all tests pass, but at least the basic frameworks work now.

Change-Id: Icea67098a8d7d58bbf918c78325681cf12a2e5f2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255313
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>