Problem: Fake LSP server does not timeout or respond to SIGTERM as it
does not run the event loop.
Solution: Instead of io.read(), use stdioopen()'s on_stdin callback to
accumulate input and use vim.wait() to wait for input.
Also, in the test suite, don't stop a session when it's not running, as
calling uv.stop() outside uv.run() will instead cause the next uv.run()
to stop immediately, which cancels the next RPC request.
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