message: Update testsuite/main.rs lint comment.
diff: -#![warn(rust_2018_idioms)] // while we're getting used to 2018
  +// See src/cargo/lib.rs for notes on these lint settings.
  +#![warn(rust_2018_idioms)]
  #![allow(clippy::all)]
  #![warn(clippy::needless_borrow)]
  #![warn(clippy::redundant_clone)]
  