Added unit tests for digger "move" functions.
Added unit tests for digger "move" functions.

New tests for
    `digger_move_left()`
    `digger_move_right()`
    `digger_move_forward()`
    `digger_move_backward()`

donmccaughey/FiendsAndFortune,donmccaughey/FiendsAndFortune,donmccaughey/FiendsAndFortune