Add tests

[fhunleth]

It would be ideal to have some form of unit testing. Things I've looked at so far:

• scr_dump - dumps the screen. Unfortunately, this is in an ncurses internal and unspecified format that doesn't look much like the current screen.
• Redirect ncurses to a regular file. This works and you can see ANSI escape sequences in addition to the text that gets written. It doesn't work for input.
• Redirect ncurses to a pipe and provide an interface to the pipe I/O to Elixir.

I think that the third option is the way to go, but I'm wondering if there's another simpler option that still exercises the NIF.