Improve unit test coverage per bug/934566.
Improve unit test coverage per bug/934566.

bug/934566 identified an error which should have been caught by
more complete unit test coverage. This review extends unit test
coverage to include handle_base_image and is one of a series I
will be sending.

Change-Id: I287fc50ea6a92239f11a107f65da84d3ff0c8b3b

n0ano/ganttclient