autotests: fix the failures I got here.
- some of them can't run in parallel
- kconfigskeletontest failed due to KSharedConfig being used under the
scene (for a year now), and there's probably some QPA or widget style
holding a ref (here, not in the CI). Indirectly, simply setting
QStandardPaths::setTestModeEnabled(true) fixes this since KSharedConfig
will then not share the instance with the non-test-mode-enabled instance.