So when multiple events are generated, the test can select one that
satisfies some condition instead of chosing 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)
        }
}
```