Tests that send signals don't work on Windows

[nathanwhit]

Originally spotted in #22757 (comment).

Many tests, such as the process.off signal test, involve sending signals to processes via ChildProcess.kill or Deno.kill. It turns out that windows doesn't really have an easy programmatic way of sending "signals", and so our implementations actually just kill the process. This means that a lot of the signal handling code for windows lacks unit tests.