ensure that events do not overlap
ensure that events do not overlap

allowing events to overlap in the tests exposes them to differences in
STL implementations. events at the same time may be processed in inverse
order on one platform from the next.

alexeymarkov/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp,ReactiveX/RxCpp