tests: Check for missing return in builtin layouts except emoji

examples/test_layout.rs

@@ -5,6 +5,7 @@ use std::env;
 
 fn main() -> () {
     check_builtin_layout(
-        env::args().nth(1).expect("No argument given").as_str()
+        env::args().nth(1).expect("No argument given").as_str(),
+        env::args().nth(2).map(|s| s == "allow_missing_return").unwrap_or(false),
     );
 }