The test suite needs to compare greyscale images and was picking an
undefined byte in the pixel data on big-endian. Select a byte that works
on any endian instead.
See: #200
As the driver is not a normal image device, we need to add a custom
script to test it. Note that the ioctl dump must also be manually
modified unfortunately as the state is tracked incorrectly for the
device by umockdev-record.
