test: Fix race condition in check-apps
test: Fix race condition in check-apps

`TestApps.testBasic` often fails like this:

    CalledProcessError: Command 'pkcon -y install appstream-data-test' returned non-zero exit status 7

because the running PackageKit still has some outdated internal state
after reconfiguring the repositories on disk. Stop it so that it
re-reads the state properly.

Closes #8907

deryni/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,cockpituous/cockpit,stefwalter/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,andreasn/cockpit,mareklibra/cockpit,garrett/cockpit,mareklibra/cockpit,martinpitt/cockpit,andreasn/cockpit,cockpituous/cockpit,andreasn/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,stefwalter/cockpit,stefwalter/cockpit,stefwalter/cockpit,garrett/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,cockpit-project/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,deryni/cockpit,mvollmer/cockpit,andreasn/cockpit,mareklibra/cockpit,andreasn/cockpit,garrett/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,martinpitt/cockpit,stefwalter/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,SotolitoLabs/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,cockpit-project/cockpit,deryni/cockpit,andreasn/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,andreasn/cockpit