Require passing unit tests

[David Edmundson]

There's a new flag we can set on CI to fail a job if unit tests fail.
https://www.volkerkrause.eu/2022/04/09/kde-ci-required-unit-tests.html

Catching things as early as possible will unquestionably be a good thing.

Realistically right now this is going to cause some merge failures as
we know some tests are flakey. It might act as good motivation to fix
them, it might just be annoying.

https://build.kde.org/job/Plasma/view/Everything%20-%20kf5-qt5/ is a
good enough overview things either yellow or blue with a cloud will
probably be issues.

Should we:
 - apply everywhere?
 - apply in a few safe places?