Improve UI test coverage

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

test_suite/tests/ui/unexpected-literal/container.stderr

@@ -0,0 +1,8 @@
+error: unexpected literal in serde container attribute
+ --> $DIR/container.rs:5:9
+  |
+5 | #[serde("literal")]
+  |         ^^^^^^^^^
+
+error: aborting due to previous error
+