Improve UI test coverage

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

test_suite/tests/ui/expected-string/byte_character.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = b'a')]
+    byte_character: (),
+}
+
+fn main() {}