tests: Fix enable-disable-stores EDS test

The test was occasionally spuriously failing because the persona stores
were being prepare()d after being re-enabled, and these prepare() calls
were being suspended part-way through when the test finished and killed
its main loop. They were being resumed when the main loop was re-started
during tear down — it’s re-started to wait for the singleton D-Bus
connection to be destroyed, so the prepare() calls ended up consistently
having their D-Bus connection drop out from beneath them, resulting in
warnings. These warnings would normally be useful to have, but not in
the middle of a unit test.

Fix this by rearchitecting the test slightly to force each persona store
to be prepared after being re-enabled. Also simplify how this is
tracked.

https://bugzilla.gnome.org/show_bug.cgi?id=726973