Unittests in sdl-sopwith project?

[Slebi]

• This pull request is more a question: Are you willing to accept unit tests in the project?
• I would like to participate in the project a bit (I know the game from when I was a child). And unit tests help me to understand the code better.
• It is my first pull request to an open source project. Feel free to reject, but if so: Please comment why you reject.

Thank you :-)