tests: Expand sound

Check Sound.duration and Sound.getDuration() for detached sounds.
Check rounding behavior of Sound.duration and Sound.getDuration()
behind a comment because NullAudioBackend doesn't respect durations.
TODO: Uncomment it once NullAudioBackend returns real durations.

core/tests/swfs/avm1/sound/output.txt

@@ -6,6 +6,9 @@ left-to-right:  0
 right-to-left:  0
 right-to-right: 100
 
+sound.duration: undefined
+sound.getDuration(): undefined
+
 Sound.setVolume(50.5)
 volume:         50
 pan:            0