Move all compile-fail tests to ui tests

The update-references.sh script makes these much easier to update in
bulk compared to compile-fail tests.

test_suite/tests/ui/unknown-attribute/variant.stderr

@@ -0,0 +1,8 @@
+error: unknown serde variant attribute `abc`
+  --> $DIR/variant.rs:12:10
+   |
+12 | #[derive(Serialize)]
+   |          ^^^^^^^^^
+
+error: aborting due to previous error
+