Prevent ViewerPluginCreate*Tests from starting Akonadi

The tests accidentally started Akonadi because the tested class
instantiates EventEdit (or Todo/NoteEdit) class, which internally
instantiates CollectionComboBox, which then starts Akonadi. This
can be workaround by passing setting the MessageViewer::_k_fooEditStubModel,
which prevents the CollectionComboBox from using Akonadi models.