Fix the way how we quit kactivitymanagerd from the unit test

Summary:
Kactivitymanagerd is a libexec executable now so we cannot just execute
it and cannot know it's path. We need to stop it as otherwise ctest does
not terminate the testActivities. This change uses dbus to quit the
kactivitymanagerd.

Test Plan: ctest did not timeout on testActivities

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16780