rpclib: In tests, use an ersatz socketpair on windows.

Nick Mathewsonrequested to merge
nickm/arti:no-socketpair-for-you into main

Please don't merge this until it is tested on windows. (@ahf can you test?)

Closes #1831 (closed)

See commit messages for explanations and complaints, and !2758 (closed) for diagnostic history.

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