run ruby files covered as tests also within emacs

* introduce a function called `cabbage-run-single-ruby-file` which first clears the buffer of the file when already exists, furthermore it compiles the ruby file.
* use the `ruby-compilation-run` function to run ruby files covered as `_test.rb` within emacs.

@senny This is just a suggestion, what do you think about it, maybe you see a better solution. :bee: