Tests get stuck when using `CALLBACK_CODE_YIELD`

[alexcrichton]

Steps to reproduce this are slightly involved:

1. Check out wit-bindgen and [patch] to use it
2. Replace this loop with self.poll()
3. Run cargo test -p wasmtime-wasi -- p3::sockets::p3_sockets_tcp_bind
4. This test should hang indefinitely

Using guest-level logging I can see that the final thing that happens is the callback returns 0x1, the code for yield, and then everything seems to stop. It seems like this is likely a bug in the host and somehow the events are being delivered, but in the logs I also see prior yields returning so it's not as trivial as "this doesn't work" but it's some combination of things I don't fully understand yet.

[alexcrichton]

Looks fixed after #143