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/big_random_buf.rs

@@ -1,12 +1,9 @@
-use wasi::wasi_unstable;
-
 fn test_big_random_buf() {
     let mut buf = Vec::new();
     buf.resize(1024, 0);
-    assert!(
-        wasi_unstable::random_get(&mut buf).is_ok(),
-        "calling get_random on a large buffer"
-    );
+    unsafe {
+        wasi::random_get(buf.as_mut_ptr(), 1024).expect("failed to call random_get");
+    }
     // Chances are pretty good that at least *one* byte will be non-zero in
     // any meaningful random function producing 1024 u8 values.
     assert!(buf.iter().any(|x| *x != 0), "random_get returned all zeros");