Refactor file conflict tests

[stephannv]

Follow up #319

As Tim said and I agree:

All these tests for file conflict handling, spread across each generator, get pretty repetitious. [...] I think my preference would be not to worry about testing for conflicts in every generator spec, instead add e.g. a spec/unit/hanami/cli/commands/app/generate/file_conflict_handling_spec.rb that demonstrates the conflict handling for just one of the generators, which would be sufficient for us to know the code is doing what it needs.