Fix incorrect stub for diffbuilder.render

Bug: Issue 11828
Change-Id: If2a8a0fa55f3aa16ae6ba7da2c82ce2ee893387e

polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.html

@@ -762,9 +762,9 @@ limitations under the License.
         element = fixture('basic');
         renderStub = sandbox.stub(element.$.diffBuilder, 'render',
             () => {
-              Promise.resolve();
               element.$.diffBuilder.dispatchEvent(
                   new CustomEvent('render', {bubbles: true, composed: true}));
+              return Promise.resolve({});
             });
         const mock = document.createElement('mock-diff-response');
         sandbox.stub(element, 'getDiffLength').returns(10000);