Improve UI test coverage

With this commit I believe I've covered all `compile_error!`-based
errors.

test_suite/tests/ui/rename/variant_unknown_rename_rule.rs

@@ -0,0 +1,14 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+enum S {
+    #[serde(rename_all = "abc")]
+    V {
+        name: u8,
+        long_name: u8,
+        very_long_name: u8,
+    },
+}
+
+fn main() {}