Bug 1069380 - Use link role instead of button for open action in cards view

Changes the button role on the screenshotView to link, and adds the aria-hidden=true attribute to the iconButton to hide it from AT's. Added tests to ensure the aria properties in the cards view.

https://bugzilla.mozilla.org/show_bug.cgi?id=1069380
I think this is unnecessary, since the selector already finds button elements. I would get rid of it + the declaration on L88.