Specifya link_args for linking libwxc into test crate to fix master build.

 * Following rust merge exposes wxRust's bug.
  * https://github.com/mozilla/rust/commit/b5c1b48048cc911c3caa07ada776239123a97b50
 * Rust doesn't propagate link_args attribute from libwx to test crate.
 * So we need to specify that explicitly.