run-test: rework the redirection script for python on windows
This should get use something something overall simpler and clearer. Especially,
we now have a `python.exe` script (even if by default Windows has
`C:\Python27\python.exe` hardcoded in the register to open .py file)
Differential Revision: https://phab.mercurial-scm.org/D11081
(binary:application/octet-stream)