Rule 3.12 verification in subscriber whitebox now really checks it

This PR reimplements the whitebox `spec312_cancelMustRequestThePublisherToEventuallyStopSignaling`, as @danarmak correctly noticed the previous implementation was not testing what it claimed it did.

Please review @danarmak, @viktorklang and @reactive-streams/contributors.
I may be a bit jet-lagged, please verify if this makes sense... :-)

Resolves #115
Can't we test for a more specific Exception here?