rusticl: specify allowed lints for tests in lib.rs

Sadly, meson doesn't allow us to specify custom flags for test targets yet

Acked-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35188>

src/gallium/frontends/rusticl/lib.rs

@@ -1,5 +1,8 @@
 // Copyright 2020 Red Hat.
 // SPDX-License-Identifier: MIT
 
+#![cfg_attr(test, allow(dead_code))]
+#![cfg_attr(test, allow(unused_imports))]
+
 mod api;
 pub mod core;