Make --test mode run without spawning kactivitymanagerd

Summary:
This allows us to run the plasma-tests repo on your system without it
destroying all your real activities. Even though one run tests in a
nested
dbus sessions, kactivities will still load and meddle with your real
database
at the same time as the real daemon is. That gets messy and broken.

It also allows us to remove a hack in the plasma-tests script.

Test Plan:
Ran normal plasma, everything is the same.
Ran plasma-tests everything there now works properly

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6133