[Plot] Add specs for PlotPanZoomStackGroup

Add specs to verify the behavior of PlotPanZoomStackGroup,
which is responsible for synchronizing the domain axis
between multiple sub-plots in a stacked plot view.
WTD-625.

platform/features/plot/src/elements/PlotPanZoomStackGroup.js

@@ -38,7 +38,7 @@ define(
 
             function clearPanZoom() {
                 stacks.forEach(function (stack) {
-                    stack.popPanZoom();
+                    stack.clearPanZoom();
                 });
             }