Auto merge of #4393 - burtonageo:update-cargo-new-template, r=matklad

Update lib template for `cargo new` so that the placeholder test contains an assertion

When you run `rustfmt` on the default generated lib template, it modifies it as shown [here](https://play.rust-lang.org/?gist=bd44610731e26e76719e75d47e598990&version=undefined)