Fix grabbing urls from srcset attributes

My first attempt at this (312ecd9) was too naive, it didn't take urls
with commas in them into account. In this second attempt, I've tried to
tdd things so that I know we can handle a wider range of values.

I've used https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img
as a guide into figuring out possible values.

packages/happo-target-firefox/src/__tests__/getUrlsFromSrcset-test.js

@@ -0,0 +1,43 @@
+import getUrlsFromSrcset from '../getUrlsFromSrcset';
+
+it('handles invalid values', () => {
+  expect(getUrlsFromSrcset('      ')).toEqual([]);
+});
+
+it('handles single urls without descriptor', () => {
+  expect(getUrlsFromSrcset('/foo.png')).toEqual(['/foo.png']);
+});
+
+it('handles single urls with commas', () => {
+  expect(getUrlsFromSrcset('/foo,40,50.png')).toEqual(['/foo,40,50.png']);
+});
+
+it('handles single urls with a width descriptor', () => {
+  expect(getUrlsFromSrcset('http://foo.com/foo.png 200w')).toEqual(['http://foo.com/foo.png']);
+});
+
+it('handles single urls with a pixel density descriptor', () => {
+  expect(getUrlsFromSrcset('http://foo.com/foo.png 1.2x')).toEqual(['http://foo.com/foo.png']);
+});
+
+it('strips whitespace', () => {
+  expect(getUrlsFromSrcset(`
+    /foo.png`)).toEqual(['/foo.png']);
+});
+
+it('handles multiple urls', () => {
+  expect(getUrlsFromSrcset('/foo.png 200w,/bar.png 400w')).toEqual(['/foo.png', '/bar.png']);
+});
+
+it('handles multiple urls with commas', () => {
+  expect(getUrlsFromSrcset('http://foo.com/foo,40,50 200w,/bar,20.png 400w')).toEqual(['http://foo.com/foo,40,50', '/bar,20.png']);
+});
+
+it('handles multiple urls with whitespace', () => {
+  expect(getUrlsFromSrcset(`
+  http://foo.com/foo,40,50 200w,
+  /bar,20.png         400w
+
+
+  `)).toEqual(['http://foo.com/foo,40,50', '/bar,20.png']);
+});

packages/happo-target-firefox/src/getUrlsFromSrcset.js

@@ -0,0 +1,10 @@
+const SRCSET_ITEM = /([^\s]+)(\s+[0-9.]+[wx])?(,?\s*)/g;
+
+export default function getUrlsFromSrcset(value) {
+  const result = [];
+  let match;
+  while (match = SRCSET_ITEM.exec(value)) { // eslint-disable-line no-cond-assign
+    result.push(match[1]);
+  }
+  return result;
+}

packages/happo-target-firefox/src/waitForImagesToRender.js

@@ -1,4 +1,5 @@
 import findBackgroundImageUrls from './findBackgroundImageUrls';
+import getUrlsFromSrcset from './getUrlsFromSrcset';
 
 export function waitForImageToLoad(url) {
   return new Promise((resolve, reject) => {
@@ -23,10 +24,7 @@ export default function waitForImagesToRender() {
         return;
       }
 
-      srcset.split(/,\s*/).forEach((entry) => {
-        const [url] = entry.split(' ');
-        promises.push(waitForImageToLoad(url));
-      });
+      promises.push(...getUrlsFromSrcset(srcset).map(waitForImageToLoad));
     });
 
     Array.prototype.slice.call(document.body.querySelectorAll('*'))