Add spec/ support for use_cases

Simply rearrange the current names/structure of the spec/ directory
Refactor directory structure

Also, fix specs that broke because of directory structure changes

The directory structure now looks like this:

doc/
  any_documentation.md

entities/
  entity_implementations.rb

lib/
  entities.rb                      # loads all entities in ../entities/
  lendger-core.rb                  # loads everything
  use_cases.rb                     # loads all interactors in ../use_cases/

spec/                              # mirrors the project's root directory
  entities/
    entity_implementations_spec.rb
  use_cases/
    interactor_implementations_spec.rb

use_cases/
  interactor_implementations.rb