Support `UNIXSocket` on Windows
In recent versions of Windows, the required parts are now supported: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/
This enables UNIXSocket, UNIXServer and UNIXSocket.pair on Windows. The anonymous socket is emulated on Windows using a temporary file, but it's good enough and the best we can do. The semantics is not completely identical.
Because these constants are now available, more tests are run on Windows.