Add QUnit test for conversion of WebP images

[Jaifroid]

Following on from #670 (comment), once #672 is complete, we should add a QUnit test for WebP image reading:

It would be good to add a QUnit test on feedNodeWithBlob function : it could create a DOM image element, call this function with a webp content (like the sample image with 2 pixels height), and the check its height afterwards.
This might probably need to refactor this function a bit : add a callback parameter or make it return a Promise, so that the unit test can know when the image has been decoded.

[kelson42]

The value of such a unit test decreases rapidly over time because less and less people use a Browser without native webp support.

[mossroy]

This should be superseded by #748 that should have a UI test that checks that an image is displayed (and check its dimensions).
If we keep running this scenario on browsers that do not support webp (IE11, Firefox 45), it will check that conversion of webp images work.
I agree with you that it would be enough.