Add helper for tests to expect image results.

[mhoff12358]

Currently still based around using image hashes but will be improved later.

BUGS=[402474472]

[mhoff12358]

@bejado I haven't gone through and updated the style comments from the last PR for this one, and there's an error with gmock on windows, so it's still a draft. But PTAL at the actual EXPECT_IMAGE calls in test_Blit.cpp and lemme know if you have any thoughts.

I'm considering whether it's worth bundling the flushAndWait(); into ImageExpectations's destructor.

Also, how do you feel about the technically redundant {} wrapping the ImageExpectations scope given that it's at the end of the test. I tend to like it because it makes it clearer that there's an object whose destruction matters. And it means that if a test needs two you don't wind up with some in enclosed scopes and some not.