How do I test my bot against other bots, like the house bots?

In the Test arena, any of your bots can be an opponent: drag in two different bots, two copies of the same one, or whole teams. To practise against a house bot, give one of your bots a house program: in Brain code, open the Start from… menu and pick House bot: and a name. To play another player's bot, challenge them from the ladder to an unrated friendly match.