Add support for `poll_oneoff` calls which just sleep on a relative timeout. This fixes a bug handling code compiled with WASI libc's `sleep` family of functions, which call `poll_oneoff` with a `CLOCK_REALTIME` timer, which wasn't previously implemented.
This is the wasi-tests crate, which contains source code for the system-level WASI tests.
Building these tests requires that the wasm32-wasi target be installed.