Tests for executability of "prime"

less ambitious settings;

future_job: more robust Exn.capture outside thread attribute change;

shutdown future scheduler after (failed) SAT_Examples, to workaround interference problem with follow-up theory loading;

future_job: tight scope for interrupts, to prevent shooting ourselves in the foot via cancel_group; control combinators: paranoia eta-expansion to ensure proper operational semantics;

maintain Future.worker_group as management data; cancel_thy: refer to proper task group; tuned;