Add tests for custom_code_classes_in_docs feature

tests/rustdoc-ui/feature-gate-custom_code_classes_in_docs.rs

@@ -0,0 +1,5 @@
+/// ```{class=language-c}
+/// int main(void) { return 0; }
+/// ```
+//~^^^ ERROR 1:1: 3:8: custom classes in code blocks are unstable [E0658]
+pub struct Bar;