public void RequestExchangeCoins()
{
var initialCoins = _testModel.Coins;
var initialCrystals = _testModel.Crystals;
var promise = _testModel.ExchangeCoinsToCrystals(10);
Assert.IsTrue(!promise.IsCompleted);
_timeModel.RealTimeSinceStartup.ReturnsForAnyArgs(4);
_testModel.Update();
Assert.IsTrue(promise.IsCompleted);
Assert.IsTrue(!promise.IsFaulted);
Assert.IsTrue(_testModel.Coins == initialCoins - 10);
Assert.IsTrue(_testModel.Crystals == initialCrystals + 10);
}