Add ui test of unimplemented trait required by dependency

test_suite/tests/ui/unimplemented/required_by_dependency.rs

@@ -0,0 +1,6 @@
+struct MyStruct;
+
+fn main() {
+    serde_test::assert_ser_tokens(&MyStruct, &[]);
+    serde_test::assert_de_tokens(&MyStruct, &[]);
+}