When a publisher's source must be asked if it's completed, we may
need to have more demand than 1 here.
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”.
checking 3.9 in PublisherVerification

+ better error messages on expectThrowingOf
Fixed 2.5 tests