Option to play the AI against itself in the client (in test mode only)

Currently, at least one human player must be in a match within the client. An option to play the AI against itself from the pull-down would be interesting for testing decks/cards used by the AI. Only accessible when testmode is true.

Start by tracing how the client creates a match, how the pull-down options are populated, and where testmode is checked. Done means a testmode-only option can start an AI-versus-AI match for testing AI decks and cards.

Written by the indexing model from the issue text.