=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
True, thanks for keeping an eye out! I'll adjust to be so then