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

[srowen]

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

[SparkQA]

Test build #26023 has started for PR 4180 at commit 43da66f.

• This patch merges cleanly.

[SparkQA]

Test build #26023 has finished for PR 4180 at commit 43da66f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26023/
Test PASSed.

[JoshRosen]

LGTM, so I'm going to merge this into master (1.3.0). I've run into this same issue with semaphore serialization and ended up applying the same fix.