Test for sync block without inputs doesn´t stop


I’m trying to make a test for a GNUradio sync block without inputs. When
I run the python qa-test it doesn’t stop, it just loops forever. When I
add an input to the block (change the io-signature and xml-file) the
test only runs once and then stops.

Is this behavoiur as it should be?
How can I make a test that terminates?