Add a wait for src of img for test_gallery

This is an attempt to address issue #296. I have watched the test run and I can see what the concern is, but I think what is happening is just a lag in how long it takes the device to render the image. I have examined the html both while the image is rendering and after it is done, and I cannot find anything that we could wait for (the html is identical). Perhaps there is an event on the phone that we could wait for, but barring that I'm not sure what else to do.

I have added an additional wait for the img src not to be blank, although that is already covered by an assertion, so there might not be any value to this update.

Let me know what you think.