Refactor test for Tennis::Game.wins_ball, add passing Tennis::Game.wins_ball method.
Add passing test 'when points is 2'.
Add passing tests for all pending Tennis::Player.score tests.
Add passing test for 'sets opponent for each player'.
Add pending tests for 'advantage', 'duce', and 'win' scores.
Add failing tests for Tennis::Player.score for 'advantage', 'duce', and 'win' scores.
Fix test for Tennis::Player.score to return 'duce'.
Add Tennis::Player.score rules to correctly return 'advantage', 'duce', and 'win'.
Refactor Tennis::Player.score to use private methods for the score tests.