consider passing --cfg test by default

[durka]

Regular cargo clippy doesn't check #[test] functions because it can't (the code is gone by the time the lints run). But if you do cargo clippy -- --cfg test it works. I wonder if there are any downsides to making this the default behavior.

[durka]

I guess this breaks people who have #[cfg(not(test))] on things. But perhaps cargo-clippy can support a --test flag which would be a bit shorter than -- --cfg test.