[SPARK-5074][Core] Fix a non-deterministic test failure

[zsxwing]

Add assert(sc.listenerBus.waitUntilEmpty(WAIT_TIMEOUT_MILLIS)) to make sure sparkListener receive the message.

[SparkQA]

Test build #25032 has started for PR 3889 at commit e61c198.

• This patch merges cleanly.

[SparkQA]

Test build #25032 has finished for PR 3889 at commit e61c198.

• 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/25032/
Test PASSed.

[pwendell]

Thanks for looking at this, I'll pull it in.

[rxin]

Thanks - merged this in master.

[rxin]

(Oops Patrick I didn't see your comment earlier)