Start to update the wasi crate in wasi tests (#675)

* Move `wasi` to `wasi_old` in wasi-tests

Leave space for the new `wasi` crate but allow us to incrementally
update tests.

* Update the big_random_buf test

* Update clock_time_get test

* Update close_preopen test

* Review comments

* Update to latest Wasmtime API

crates/test-programs/wasi-tests/src/bin/sched_yield.rs

@@ -1,4 +1,4 @@
-use wasi::wasi_unstable;
+use wasi_old::wasi_unstable;
 
 fn test_sched_yield() {
     assert!(wasi_unstable::sched_yield().is_ok(), "sched_yield");