Create a custom player match, setting the max players to two (or up to four) and the game to Team CTF with the captures set to three. Then turn off auto-balance and have both players on the same team. Take turns running the flag to score and re-launching the game afterward.
Warning: For some reason, the game likes to reset wins or just forget about some of them, so it is suggested to do all 250 wins in one sitting, or no more than two sittings. It will take a while and be very boring, but if you're taking turns it shouldn't be too bad.