Improve UI test coverage

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

test_suite/tests/ui/expected-string/float.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/float.rs:6:22
+  |
+6 |     #[serde(rename = 3.14)]
+  |                      ^^^^
+
+error: aborting due to previous error
+