Modify testPauseOnBackButton to do something useful

When entering the stage assert that the look size is the default size.
Pause the script by pressing ```goBack```.
The ```WaitBrick``` would wait for 5 seconds so after 6 seconds on pause check for the size again. If the script is on pause as specified, the size would still be the default size.
Resume the stage and wait again 6 seconds. The ```WaitBrick``` will have run by now and the size now is at 50%.

Jenkins (single-UI-emulator) testrun: https://jenkins.catrob.at/job/Catroid-single-UI-emulator/337/

Continue #750 
Fix #722