tests: add a module that can perform the equivalent of `SIGKILL` on any OS I started with this being Windows specific, but let's push all of the decision making into this function so that it can just be called by the tests. The tradeoff is that this is very specific to sending `SIGKILL`- since `signal.SIGKILL` doesn't exist on Windows, the desired signal can't be passed from the caller. Maybe there's a way, but let's wait until there's a need. We don't use `killdaemons.py` unconditionally because it starts with a more graceful `SIGTERM` on posix.