add error message when #[naked] is used with #[test]

compiler/rustc_error_codes/src/error_codes/E0798.md

@@ -0,0 +1,14 @@
+Testing attributes cannot be applied to functions marked with `#[naked]`.
+
+Erroneous code example:
+
+```ignore (requires test runner)
+#[test]
+#[should_panic]
+#[naked]
+fn foo() {}
+```
+
+See [the reference page for testing attributes] for more information.
+
+[the reference page for testing attributes]: https://doc.rust-lang.org/reference/attributes/testing.html

compiler/rustc_error_codes/src/lib.rs

@@ -536,6 +536,7 @@ E0794: 0794,
 E0795: 0795,
 E0796: 0796,
 E0797: 0797,
+E0798: 0798,
         );
     )
 }