rfc2: add regression test for mirb and mruby binaries

this pr is an alternative to #1578

it reuse the test framework for unit test in mruby, but is executed in cruby to take advantage of sh operations, as @bovi suggested, someone might want test mruby without bash.

![screen shot 2013-11-16 at 3 11 34 pm](https://f.cloud.github.com/assets/129800/1555819/cf3d641e-4e8d-11e3-9a78-9648ee272954.png)