Image comparison should be done on image data, not file data

[reticulatedpines]

When we test emulation by comparing screenshot capture to expected image file, we currently compare file hashes. This has been reliable so far, but could be annoying if e.g. vncdotool starts using a different PNG header. We'd fail even though the images looked identical.

It would be better to compare the image data, this is conceptually correct and should be more robust.

Current code is this in tests.py:

            for k in key_sequence:
                capture_filename = q.key_press(k)
                capture_filepath = os.path.join(self.output_dir, capture_filename)
                expected_hash = 0
                with open(capture_filepath, "rb") as f:
                    test_hash = hashlib.md5(f.read()).hexdigest()