Adapted tests for slider classes with ExtJS5

This PR adresses the tests for the classes ``GeoExt.slider.Zoom`` and ``GeoExt.slider.LayerOpacity`` to make them work with the new ExtJS5 mechanism.
Especially the commit 3887196b47126440f3256fd26a182dd5232b337d is worth a review because this changes the source code of the two mentioned classes.