test: five assertions print what they saw when they fail

[DevanshGang]

What & why

Impoves the assertion failure message in five tests so that test failures provide clearer context and are easier to diagnose.

Closes #545

Checklist

• Linked an issue (or explained above why none exists)
• Tests added/updated for the change
• cargo fmt --all and cargo clippy --workspace --all-targets --all-features are clean
• All commits are signed off (git commit -s) — see CONTRIBUTING.md §5
• No AI attribution trailers (no AI co-authors, "Generated with" footers, or bot identities) — see CONTRIBUTING.md §6
• CHANGELOG.md updated under [Unreleased] (user-facing changes only)
• Snapshot changes (if any) were reviewed with cargo insta review, not blind-accepted

[vyncint]

Thanks again! Four of the five were right as written, and binding the result to a variable before the matches! is exactly the shape the builder_validation and compat ones needed.

I pushed one commit on top. {:?} on none.stdout prints the Vec as byte values, [117, 115, 97, ...], rather than text, so it now goes through String::from_utf8_lossy like the stderr assertion right below it. And the timeout check prints {err}, as the issue suggested: the Display form is the readable one, with the screen block. I also retitled the PR to Conventional Commit form (test: …, no space before the colon), since the title becomes the commit subject on main. Merging.