Improve UI test coverage

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

test_suite/tests/ui/rename/container_unknown_rename_rule.rs

@@ -0,0 +1,12 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(rename_all = "abc")]
+struct S {
+    name: u8,
+    long_name: u8,
+    very_long_name: u8,
+}
+
+fn main() {}