Improve browser tests

- Listen for image change event instead of waiting 200 ms
- Properly fake fetch response, can never be undefined on resolve

test/browser/spec/ol/layer/MapboxVector.test.js

@@ -208,7 +208,7 @@ describe('ol/layer/MapboxVector', () => {
       originalFetch = fetch;
       window.fetch = function (url) {
         fetchUrl = url;
-        return Promise.resolve();
+        return Promise.resolve({ok: false});
       };
     });
     afterEach(function () {