new function to run a single ruby test-file. WIP

Key concepts:
  - no buffer flickering
  - no buffer switching
  - clear after each run
  - support minitest and test-unit

Work to do:
- [x] don't switch to the test output buffer when executing a test
- [x] the test buffer should stay at the same place once it is open
- [x] set the `default-directory` to the project root of the last executed test
- [x] fix blank test output windows

> Was caused by a bug in `trunk` and is not related to the code of this PR

- [x] support test-suites that don't use `require_relative` and need the test directory in the `LOAD_PATH`

> `cabbage-ruby-test-ruby-options` allows to pass options to `ruby` and defaults to `-Itest`

- [x] support test-suites that need to pass environment variables

> Can be achieved using `(setenv "ENV" "VALUE")`

- [ ] somehow detect if `_spec.rb` is rspec or `Minitest::Spec`. Not sure how though...