Auto merge of #37826 - keeperofdakeys:proc-macro-test, r=alexcrichton

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.

diff --cc src/librustc_driver/driver.rs
Simple merge