test: Add initial testing for the Empathy game

Two general techniques that are introduced here that will be useful
for testing of other games:

  * Registration of multiple user and events sent from each (see
    multiple cookies and a player-cookie parameter for many empathy_*
    functions)

  * Fishing events out of the event stream in response to client
    requests (see curl_get_event)

This testing gets as far as is currently implemented in the server:
Multiple player join, a category is submitted and voted one, the game
starts, all players submit answers, players receive words to judge,
and players submit word groups to capture the results of their
judging.

What's not yet tested is the next phase that's also not implemented
yet: The server using the submitted word groups to affect the scoring.