Ctrl-C doesn't always kill all python processes

[timwoj]

This happens on all platforms but it's especially broken on Windows. Pressing ctrl-c on Windows when running with a -j argument will kill one of the currently-running tests, but will continue to run the others and spawn new ones. Pressing it repeatedly will eventually kill all of the currently-running tests but will then hang the entire process. This leads to having to open Task Manager to manually kill all of the python processes.