Divide test_filter players_by_event into smaller tests in game_event_player/tests.pay

[amitkaplansky]

To improve readability, there’s a need to divide this test into three functions:

1. function for creating GameEventPlayers.
2. A function that will test whether a player is in the expected events.
3. A function that will test whether an event has the expected players.

In this way every function will be smaller, easier to read and understand.