Improve UI test coverage

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

test_suite/tests/ui/transparent/with_from.stderr

@@ -0,0 +1,11 @@
+error: #[serde(transparent)] is not allowed with #[serde(from = "...")]
+ --> $DIR/with_from.rs:5:1
+  |
+5 | / #[serde(transparent, from = "u64")]
+6 | | struct S {
+7 | |     a: u8,
+8 | | }
+  | |_^
+
+error: aborting due to previous error
+