Windows support

[deathowl]

Testslide historically does not support Windows. Figure out why, and make it work there too.

[fornellas]

IIRC I gave it a try at some point, but there were 2 bumps on the road:

• IIRC there was some odd ball test breaking on Windows (due to stupid stuff, not hard to fix).
• Missing Windows build on CI to get signal.