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/fd_readdir.rs

@@ -1,7 +1,7 @@
 use libc;
 use more_asserts::assert_gt;
 use std::{cmp::min, env, mem, process, slice, str};
-use wasi::wasi_unstable;
+use wasi_old::wasi_unstable;
 use wasi_tests::open_scratch_directory;
 use wasi_tests::wasi_wrappers::{wasi_fd_filestat_get, wasi_fd_readdir, wasi_path_open};