Test fix

Tiny fix to the test to account for URLs that may have had uppercase
letters (the audio embed converts to lowercase to account for links to
.MP3 files rather than .mp3)

src/sidebar/test/media-embedder-test.js

@@ -109,7 +109,7 @@ describe('media-embedder', function () {
 
       assert.equal(element.childElementCount, 1);
       assert.equal(element.children[0].tagName, 'AUDIO');
-      assert.equal(element.children[0].src, url);
+      assert.equal(element.children[0].src, url.toLowerCase());
     });
   });