So here is a patch to apply to test/unit
It is really experimental and needs more work, but it is already functionnal.

http://flwrpwr.free.fr/Ruby/testunit_hl_console.diff

apply with:

cd ruby-1.8.1/lib
patch -p0 < path_to/testunit_hl_console.diff

and you will have to reinstall test/unit.

Hope it will help.

Denis