Guide: refresh Testing

[chastell]

This refreshes the Cargo commands’ output for the ‘Testing’ chapter to match current reality.

This also drops the ‘Wait a minute, zero tests?’ paragraph, which is no longer relevant when one actually follows the steps in the Guide.

Also, when following the steps in the Guide dilligently, the warning about unused main does not show up again, as src/main.rs is not recompiled when unchanged; these changes address this as well.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

[steveklabnik]

Awesome. This kind of change is the worst to keep up with. Thanks so much!

[chastell]

Argh, you were too fast – I noticed that Cargo not only added the bang to ‘Hello, world!’, but also dropped the semicolon. I updated the relevant commit and rebased, but this removed any ‘r+’ given.

Apologies for the extra work, I didn’t realise the approval process is commit-based. I’ll be more careful in the future. 😅

[chastell]

(I only removed the semicolon from the Cargo-generated code and its derivatives, but kept it in the first few hand-written examples that actually mention the semicolon. I think this way it raises fewer questions when the Guide is read top-down.)