test_timeout fails on Mono

There appears to be two failures (only tested on Linux but I assume the issues are the same on macOS)

1. The timeout values (in Socket.SendTimeout) only being stored in tens of milliseconds:

>>> import _socket
>>> s = _socket.socket()
>>> s.settimeout(1.234)
>>> s.gettimeout()
1.24

2. When certain operations timeout (e.g. recv), the SocketException.SocketErrorCode is SocketError.WouldBlock instead of SocketError.TimedOut so we end up with an OSError instead of a socket.timeout. See https://github.com/mono/mono/issues/20381

Start with the test_timeout case and inspect the Socket.SendTimeout and SocketException handling involved in the reported operations. Reproduce the Mono behavior, then verify that timeout values retain their precision and that timed-out operations produce SocketError.TimedOut and socket.timeout rather than OSError.