pytest: Simplify usage.

[wxtim]

Describe the bug
At commit 1fca257d4d5 it was found necessary to run pytest separately for the doctests and tests folder.

Steps to reproduce the bug
run pytest --doctest-modules.

Expected behavior
Doctests from cylc/ and tests from tests/ should both be run.

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).

[wxtim]

I've replicated the problem without running the entire battery using

pytest cylc/rose/entry_points.py tests/test_functional_post_install.py --dist=no -n0 --pdb

Looks like somewhere code is importing rose without the leading metomi, but it's not clear why.

[MetRonnie]

According to the traceback it's in Rose

https://github.com/metomi/rose/blob/1734ce317ff2cc08d6f2782b119dddc10df0e3fb/metomi/rose/config_processor.py#L112-L113