minitest/unit.rb: defer requiring -test-

* test/lib/minitest/unit.rb (find_fds): defer requiring a library
  under "-test-" until needed, or can't work in other (installed)
  ruby.  grep doesn't need separated map.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46248 b2dd03c8-39d4-4d8f-98ff-823fe69b080e