Don't select a shared calendar as schedule-default-calendar-URL, fix for #550

Quite easy to fix as we already have the shared-info in the resource type. As @evert mentioned in #550, I also don't think we really need to check for users having write-access on the calendar as it is currently not possible to create an own read-only calendar (if I didn't miss something).
The calendar backend is completely created from scratch every time we hit setUp().

Most of the logic for that is handled in the parent class.