[ruby-mode] Added some tests for ruby config.

What do you think of the few tests that I have written?

If they look fine to you I'm gonne keep on writing some more with all the use cases of all the pull requests I have made so far.

Otherwise, let me know what you would have done differently.

My idea is to make it easy to write tests, so I made a little DSL that creates a temp buffer with the input string, moves the point to `X`, removes `X` then run the given command n times and compare the result with what's expected.