Closes #325

Only run clean up tests in controller worker to prevent cleanup errors: Flaky tests were being caused by errors in cleanup when tests were distributed and some workers attempted to cleanup before other workers were finished with the same fixture.

Tested by running:

# Crank n up as high as you think that you can get away with.
> for i in $(seq 1 50); do pytest -n 24 >> repeater.txt; done

# 🍵 🛌🏼 🌅  (Go have a cuppa, or leave it overnight)

> grep '224 passed in' repeater.txt | wc -l
...
> grep Error repeater.txt
# Hopefully nothing

I have also checked that

# Master
du -h --max-depth=0 /var/tmp/pytest-of-tpilling/pytest-1746/popen-gw0
316K	/var/tmp/pytest-of-tpilling/pytest-1746/popen-gw0

# This branch
du -h --max-depth=0 /var/tmp/pytest-of-me/pytest-1747/popen-gw0
316K	/var/tmp/pytest-of-me/pytest-1746/popen-gw0

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).
• CHANGES.md 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.