lexmark-fax-pnh: Add unit test for transform() function

Increases unit test coverage to 100%.

BUG=b:248023706
TEST=cros_run_unit_tests --board ${BOARD} --packages lexmark-fax-pnh

Change-Id: If67700a101b48a99c89e542a2862f0a7ca125065
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/lexmark-fax-pnh/+/3914709
Commit-Queue: Bryan Cain <[email protected]>
Tested-by: Bryan Cain <[email protected]>
Reviewed-by: Benjamin Gordon <[email protected]>
Auto-Submit: Bryan Cain <[email protected]>
4 files changed