TestBench: Small fix

Check if the "on" signal ends with "Changed" to make
sure we are dealing with a property.

I.e. onClicked should be skipped. Also in that case
it caused the substr to be empty and the call
item[substr] to create an assert in V8.

Change-Id: Ib04aa1773ef13a005e998659c7c903d3118bfc62
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>