Don't stub out ES module imports

It is not allowed and only happens to work because babel doesn't
strictly follow the specification. It doesn't seem necessary for the
tests to run, so just remove it.

tests/test.mouse.js

@@ -1,12 +1,10 @@
 const expect = chai.expect;
 
 import Mouse from '../core/input/mouse.js';
-import * as eventUtils from '../core/util/events.js';
 
 describe('Mouse Event Handling', function() {
     "use strict";
 
-    sinon.stub(eventUtils, 'setCapture');
     // This function is only used on target (the canvas)
     // and for these tests we can assume that the canvas is 100x100
     // located at coordinates 10x10