Add a simple test for TCP sockets

[catamorphism]

Add two test programs, sockets-client.c and sockets-server.c, that are run by the test driver as separate executables (since without threads, this can't be written as a single test).

[catamorphism]

I'm not sure why the tests are failing in CI; everything works for me locally. I'll keep investigating.

[catamorphism]

The Makefile logic isn't right (although that doesn't explain why the tests fail); working on it.

[pchickey]

Thanks! We should test deeper when we get more time - want to see a server handle multiple connections in an accept loop to give coverage of poll(2) or select(2), hangups on either side, and nonblocking sockets - but this fixes the major deficiency of a smoke test.

@dicej when you have time - how can we reproduce the testing you performed on p2 sockets when that first landed?

[dicej]

@dicej when you have time - how can we reproduce the testing you performed on p2 sockets when that first landed?

https://github.com/dicej/wasi-sockets-tests contains various smoke tests. I also relied heavily on CPython's suite of asyncio socket tests (ignoring the failures due to things like process forking, multithreading, and signal handling, which we don't have in WASI). I confess I don't recall the details of how I ran that test suite, though; I'd have to dig through my shell history or rediscover the incantations; happy to do that if it would be helpful.

[pavelsavara]

dotnet mono also has good WASIp2 coverage for sockets, but our CI for it is currently broken and we are behind on WASI-SDK upgrades.

We only care for async/non-blocking. That allows us to play role of the server and client in the same app.