Add some more test cases to RandomAI

[cgolubi1]

• sometimes choose Default attacks, using dice which have already been chosen as valid for some attack type
• occasionally choose Surrender attacks
• occasionally intentionally submit an invalid attacker/defender combo
• after any submitTurn call fails, check that the API-reported game state is unchanged

These are additions i've had in mind to make for awhile, and the relevant code was top-of-mind because of the "invalid pass" fix. I've run about 2000 novel and replayed games, and nothing has turned up, so i think it's okay to put this up for PR.

[blackshadowshade]

Changes are only to random_ai, looks okay to me. CircleCI passes. Accepting now.