Mock PointerEvent in tests to include target

The `target` Event property is readonly as it is set internally when an
event is dispatched. This change uses a plain object with the essential
properties that a PointerEvent has which is sufficent for map event handling

test/spec/ol/interaction/modify.test.js

@@ -84,6 +84,7 @@ describe('ol.interaction.Modify', function() {
     const position = viewport.getBoundingClientRect();
     const pointerEvent = new Event();
     pointerEvent.type = type;
+    pointerEvent.target = viewport.firstChild;
     pointerEvent.clientX = position.left + x + width / 2;
     pointerEvent.clientY = position.top + y + height / 2;
     pointerEvent.shiftKey = modifiers.shift || false;