Deny warnings in the CI by default (#616)

* Deny warnings in the CI by default

This commit passes `-D warnings` flag to `rustc` in the Test stage
of the CI in order to deny warnings on _all_ tested platforms.

* Fix warnings

* Fix warnings

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

@@ -1,6 +1,4 @@
-use libc;
 use std::mem::MaybeUninit;
-use std::{env, process};
 use wasi::wasi_unstable;
 use wasi_tests::wasi_wrappers::wasi_fd_fdstat_get;