This commit updates _all_ WASI test programs to use the latest
version of the `wasi` crate (`v0.9.0`). While at it, it also
unifies asserting error conditions across all test programs.
* 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