fix test build with some feature combinations

Author: RalfJung

src/lib.rs

@@ -2,7 +2,7 @@
 #![deny(missing_debug_implementations)]
 #![cfg_attr(test, deny(rust_2018_idioms))]
 #![cfg_attr(all(test, feature = "full"), deny(unreachable_pub))]
-#![cfg_attr(test, deny(warnings))]
+#![cfg_attr(all(test, feature = "full"), deny(warnings))]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]
 #![cfg_attr(docsrs, feature(doc_cfg))]