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/unlink_file_trailing_slashes.rs

@@ -1,5 +1,5 @@
 use std::{env, process};
-use wasi::wasi_unstable;
+use wasi_old::wasi_unstable;
 use wasi_tests::open_scratch_directory;
 use wasi_tests::utils::{cleanup_dir, create_dir, create_file};
 use wasi_tests::wasi_wrappers::wasi_path_unlink_file;