Improve test harness setup

Ruby 1.9 has test/unit in stdlib (and is back-compat with 1.8.x).
Also, explicitly asking for a test runner is not necessary (and
not good practice). Running `ruby test.rb` will automatically
hook up the correct test runner UI; tools that use other runners
can add their own.
Merge pull request #6 from lsegal/test-improvements

Improve test harness setup