Update testsuite/main.rs lint comment.

Author: ehuss

tests/testsuite/main.rs

@@ -1,4 +1,5 @@
-#![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)]