move image comparison of Alert back to gfx test

This moves the image comparison of Alert back to the AlertTest gfx test, which it used to be, before we moved it to the TestAlert testlet. I didn't remove the testlet, which also tests other aspects of Alert; I just moved the part of it that does the image comparison.

I also updated the AlertTest.png image again, since its title, text, and button are coming out centered. I'm not sure why, as I don't think it has anything to do with my changes. Perhaps it's something else that has changed on the branch? In any case, I think we probably want those to be left-aligned (except maybe the button), but this way at least the gfx test passes on my machine. We'll need to update its maxDifferent to get it to pass on other machines.

(There's a merge from master here too.)