If you like test::unit or are simply stuck with it, this works great on ruby 1.8 and 1.9+, have fun 🙂

gem install testrbl

testrbl test/unit/xxx.rb:15 # test or should or context on line 15
testrbl test/unit.xxx.rb # all tests in xxx.rb
testrbl test/unit # all tests in folder

Github:
testrbl: run Test-unit + shoulda + minitest by line