Improve UI test coverage

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

test_suite/tests/ui/precondition/serialize_field_identifier.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(field_identifier)]
+enum F {
+    A,
+    B,
+}
+
+fn main() {}