validate: scenarios: set need clock sync for switching subtitle track

validate: scenarios: set need clock sync for switching subtitle track

As soon as the track is changed, the pipeline state is set to NULL
by execution 'stop' action even if there is a 'playback-time' with 5sec.
If the AV sink is not synchronized,
audio fakesink and video fakesink has different position value.
When the validate request the position information of pipeline
to do 'stop' action, the audio fakesink response of the position query
with the bigger value than 5sec.

https://bugzilla.gnome.org/show_bug.cgi?id=755101