Show a better error when using --test with #[proc_macro_derive]
Fixes https://github.com/rust-lang/rust/issues/37480
Currently using `--test` with a crate that contains a `#[proc_macro_derive]` attribute causes an error. This PR doesn't attempt to fix the issue itself, or determine what tesing of a proc_macro_derive crate should be - just to provide a better error message.