cat-log: list out/err files when available via tailer

[oliver-sanders]

Closes https://github.com/cylc/cylc-flow/issues/5802

List log files that are not present on disk but which are available via a tailer.

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.

[oliver-sanders]

Note: This approach works just fine, however, it has a couple of small caveats:

• It isn't checking whether the out/err files have appeared in the output already reported - i.e. we could get duplicate job.out / job.err files in the output.
• The job.out and job.err files will always appear at the end, not in alphabetical order.

Ideally, we would avoid these caveats. I did start looking at a solution where I added the job.out and job.err files onto the end of the mode variable, e.g. mode = list-dirs:job.out:job.err, that way they can be output along with the regular directory listing in a much more elegant way.

But I ran out of time. Writing this up in case anyone else needs to take this one over.