Fix broken uploadLastResult test

I forgot to provide a default value for the options, plus the promises
used in the test case were never returned, leading to silent failures.

src/server/__tests__/uploadLastResult-test.js

@@ -52,7 +52,7 @@ describe('when there are files to upload', () => {
       ],
     }));
 
-    uploadLastResult().then(() => {
+    return uploadLastResult().then(() => {
       // 2 times each diff, 1 times the new image, 1 times index.html
       expect(uploadMock.mock.calls.length).toEqual(6);
     });
@@ -70,7 +70,7 @@ describe('when there are files to upload', () => {
       ],
     }));
 
-    uploadLastResult().then(() => {
+    return uploadLastResult().then(() => {
       // 1 times the new image, 1 times index.html
       expect(uploadMock.mock.calls.length).toEqual(2);
     });

src/server/uploadLastResult.js

@@ -66,7 +66,7 @@ function uploadHTMLFile({ uploader, diffImages, newImages, triggeredByUrl }) {
  * @return {Promise} that resolves with a URL to the html document uploaded to
  *   s3.
  */
-module.exports = function uploadLastResult(triggeredByUrl, { debug }) {
+module.exports = function uploadLastResult(triggeredByUrl, { debug } = {}) {
   return new Promise((resolve, reject) => {
     const { diffImages, newImages } = getLastResultSummary();