Raise ambiguity in ICalFormat::toString() in tests
For some reason, CI had trouble to compile toString(calendar), making toString(KCalendarCore::MemoryCalendar::Ptr&) and toString(const KCalendarCore::Incidence::Ptr&) ambiguous. Add the optional argument to the first call so the ambiguity is raised.
The second change is because I've trouble at link time in the Sailfish SDK also, so trying to be cautious here, I'm proposing it also in case the CI may raise the same problem.