Tweak outputs logging, fix test comments (follow-up 6999)

[hjoliver]

Trivial change to follow up #6999

Can go in 8.6 if there's time, but no biggie.

• put new debug logging behind a verbosity check - Don't infer optionality from RHS implicit outputs. #6999 (comment)
• correct several comments in new tests (was a final commit on 6999 that I neglected to push) - Don't infer optionality from RHS implicit outputs. #6999 (review)

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.