Fix.ensure logger set for all caplog.set level

[wxtim]

caplog.set_level can conflict with other instances of caplog if the logger isn't set in the tests. This was causing the manylinux tests to fail, and would have caused the same problem if all pytests were run in the same task in the fast tests.

https://github.com/wxtim/cylc/actions/runs/7501317283 (PR's with changes to Python tests only trigger the fast-tests workflow - a neat feature, but somewhat unhelpful in this particular case)

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).
• This is a change to testing, no new test, documentation or changelog entries are reqd
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

@oliver-sanders pick a second reviewer if you think it required.