Async Chat Handling

[ColoredCarrot]

Currently, tests dealing with chat (like testing that a player has received a message) sometimes fail non-deterministically, always relating to a client not having received an expected message.
For examples, see ChatTest in the example module.

This happens because Spigot tends not to handle chat on the primary thread. syncPackets() might return before the clientbound chat packet has been picked up by the packet snooper, and therefore before the server-side clientbound packet count has been incremented.

AC:

• Chat-related tests have deterministic results.
• Invocations of syncPackets() guarantee that all relevant chat messages will have been sent to and received by the client.