Use the STD_FEATURES list

We define a list of features that should be tested along with "std" but
we don't actually use it. Add a call to `cargo test` that enables "std"
and all the features from `STD_FEATURES`.

Found by `shellcheck`.

contrib/test.sh

@@ -49,6 +49,7 @@ if [ "$DO_FEATURE_MATRIX" = true ]; then
     RUSTFLAGS='--cfg=fuzzing' RUSTDOCFLAGS=$RUSTFLAGS cargo test --all
     RUSTFLAGS='--cfg=fuzzing' RUSTDOCFLAGS=$RUSTFLAGS cargo test --all --features="$FEATURES"
     cargo test --all --features="rand serde"
+    cargo test --features="$STD_FEATURES"
 
     if [ "$NIGHTLY" = true ]; then
         cargo test --all --all-features