style: Green up and re-enable style system layout tests

It's unclear to me when they were disabled, but we do want to enable
these as otherwise there's no way to catch bindgen issues that can end
up in subtle bugs at best, or memory corruption at worst.

Differential Revision: https://phabricator.services.mozilla.com/D162997

components/style/gecko_bindings/mod.rs

@@ -16,6 +16,9 @@
     non_upper_case_globals,
     missing_docs
 )]
+// TODO: Remove this when updating bindgen, see
+// https://github.com/rust-lang/rust-bindgen/issues/1651
+#[cfg_attr(test, allow(deref_nullptr))]
 pub mod structs {
     include!(concat!(env!("OUT_DIR"), "/gecko/structs.rs"));
 }