Guide: refresh Testing

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.