[Piglit] Appveyor: Add support for python tests in addition to native tests

[Dylan Baker]

This little series fixes a few bugs, centralizes a few things, and adds python
tests back to appveyor, while maintaining support for native C/C++ compilation.
It does this by converting several sections to powerscript, which I find easier
to read and write than cmd (batch) scripts.

This is available at my github:
https://github.com/dcbaker/piglit wip/appveyor-hybrid