Add filtering function for data generation

[jsoriano]

So when multiple events are generated, the test can select one that
satisfies some condition instead of choosing the first one, like in this
example, that would write the first event with the messages key:

func TestData(t *testing.T) {
        f := mbtest.NewEventsFetcher(t, getConfig())
        err := mbtest.WriteEventsCond(f, t, func(e common.MapStr) bool {
                hadMessages, _ := e.HasKey("messages")
                return hadMessages
        })
        if err != nil {
                t.Fatal("write:", err)
        }
}

[ruflin]

WFG