Fix in 1.13 rule verification

113 - demand requested upfront, for EXACTLY the amount of elements the
underlying iterator contained.
FIX: The TCK can in this case signal more demand, so we actually trigger
the call to hasNext that returns false, so we throw Stop and send
onComplete. We can’t check hasNext inside the previous production call,
because we must return the “simple value” (number), here, and there is
not other way to signal “this was the last element”.
More fine-tuning of the specs, fixes of mistakes uncovered by akka-streams

+ checking 3.9 in PublisherVerification
+ better error messages on expectThrowingOf
+ fixed 2.5 tests – they were checking the wrong things
+ checking 1.13 must be optional, because some publishers dont support fan-out
+ inverse condition in 3.13 to avoid NPE, and report a nice error
+ checking 1.7 should give more demand, then pull based publishers will
notice that their source has been drained and complete the downstream