[SPARK-9948] Fix flaky AccumulatorSuite - internal accumulators

In these tests, we use a custom listener and we assert on fields in the stage / task completion events. However, these events are posted in a separate thread so they're not guaranteed to be posted in time. This commit fixes this flakiness through a job end registration callback.

Author: Andrew Or <andrew@databricks.com>

Closes #8176 from andrewor14/fix-accumulator-suite.