SPARK-960 [CORE] [TEST] JobCancellationSuite "two jobs sharing the same stage" is broken

This reenables and fixes this test, after addressing two issues:

- The Semaphore that was intended to be shared locally was being serialized and copied; it's now a static member in the companion object as in other tests
- Later changes to Spark means that cancelling the first task will not cancel the shared stage and therefore the second task should succeed

Author: Sean Owen <sowen@cloudera.com>

Closes #4180 from srowen/SPARK-960 and squashes the following commits:

43da66f [Sean Owen] Fix 'two jobs sharing the same stage' test and reenable it: truly share a Semaphore locally as intended, and update expectation of failure in non-cancelled task