Couple changes in the test:
1) add also a component into the calendar, thus the returned
   Free/Busy component is not empty
2) let the sync variant wait for the D-Bus signal with Free/Busy
   data again
3) wait for the D-Bus signal in case it's not received in time
   of the method execution finish.

Especially the 3) is important, because the D-Bus signal can be delivered
after the method invocation is received, which could break the test.