Command factory tests

I was reviewing the project code to check which classes can be tested easily (#44), but I'm afraid there will be bigger refactoring steps in the future before many classes of the project can be tested.

For now, I contribute small tests of the command factory, creating build in and custom commands. For custom commands, I created some dummy commands to be able to check exceptions and returned command instances.

I guess next time will be a PR with refactored code.