Updated 'SchedulerTest.TaskGroupRunning'.
Updated 'SchedulerTest.TaskGroupRunning'.

This patch updates the test `SchedulerTest.TaskGroupRunning`
to confirm that the agent-side code responsible for launching
task groups works correctly. Previously, this test only
verified that the `RunTaskGroupMessage` was sent to the agent.

Review: https://reviews.apache.org/r/57807/

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