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
Right, had a counter here actually but removed it (jet-lag might be showing...).
I'll add it back (as method, so it's configurable, in case impl. does "batches of 1000" etc).