BuildCommand tests

[devmsh]

In order to test the BuildCommand, I changed some code to make the Builder and BuildCommand more testable

1. Inject Builder to BuildCommand nested of Static method invocation to unit test the command. We can also provide e Builder facade and return the static method invocation if needed.
2. Replace file_exists with $this->files->exists to allow tests code to mock it.
3. Replace exit(1) with return 1 so we can use assertExitCode(1) and prevent the command from killing the test command.
4. Simplifying defaultDraftFile is not related to the test code, and I can revert the change if you like the old style.

Waiting for your feedback, and if everything goes ok, please select the next candidate to be tested.

Have a nice day 👍

[jasonmccreary]

Seems good. I'm going to merge this. But watch for the code style failure next time.

Feel free to continue adding tests for the commands.