=tck #153 spec317 test must explicitly check the error was asserted on

Fixes the `spec317_mustSignalOnErrorWhenPendingAboveLongMaxValue` which was added for `1.0.0.M1` to explicitly check for triggering the overflow onError signal as well as giving more breathing room to the implementations (avoid escaping the loop and even the entire test before the onError triggers).

Resolves https://github.com/reactive-streams/reactive-streams/pull/154
The push swallowed @viktorklang's comment, so quoting here from github's email:
> If it's the test-runner thread and it does not impact the publisher or subscriber, I'm not that worried about it.
Correct, it's the test runner. The subscriber will be signalled in the background if it's an async publisher, In the sync publisher case well - everyone sleeps, there's just 1 thread - but that also means it's not interfering as there is no other action going on.