Add explicit test/unit dependency

This change needs consultation with CRuby guys.

 * CRuby trunk and 2.2.0-preview1 needs this but is this intentional
   change?
 * If this change only needs for developers living with CRuby trunk?

FWIW I once heard that this is only needed by the ruby command built
from svn repo but unnecesarry in general ruby installation.
Merge pull request #442 from nahi/topic_explicit_test_unit_dependency

[WIP] Add explicit test/unit dependency