It is not very useful to just delete the data again after a failure, as
it might be useful for debugging. Just store it into an "errors"
subdirectory of the PWD in the hope that this is a sane location.
Note that it'll error out if the directory already exists, but that
should be acceptable in all cases. i.e. it won't exist in the CI and
developers can just wipe the directory.
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.
