dummyssh: call python script with python
We don't want windows to grab whatever python got configured to run .py file. We
need the python used during the test to be used.
Differential Revision: https://phab.mercurial-scm.org/D11083
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