Add unit test suite (for use at build time and for autopkgtest)

This commit adds the unit test script test_dh_octave.sh, which is
based on shunits2 and exercise some trivial tests involving the
commands dh_octave_clean, dh_octave_changelog, and dh_octave_version.

This script can evolve in the future for testing more subtle aspects
of the dh-octave scripts, including the more complex ones
(dh_octave_make and dh_octave_check).