[#59] add tests for catching global and local parser conflicts

[zetkez]

Resolves #59

I added a few tests to demonstrate the conflicts between the local and global parser. The tests are by no means exhaustive, especially the tests using a optparse-applicative command. I put everything into the existing files and functions and now the code looks a bit crammed to me. Would it make sense to transfer it to a new file or at least create a new :: Spec function? Also i did some tiny refactoring.

First time ever contributing to a Haskell project and feedback is much appreciated.

Additional tasks

• [ ] Documentation for changes provided/changed
• [x] Tests added
• [ ] Updated CHANGELOG.md

[zetkez]

@chshersh i moved the tests to its own spec function and moved the code down inside the file. I sadly managed to loose your review comments while doing a force push.