Fixed test `MesosContainerizerSlaveRecoveryTest.ResourceStatistics`.
Fixed test `MesosContainerizerSlaveRecoveryTest.ResourceStatistics`.

`MesosContainerizerSlaveRecoveryTest.ResourceStatistics` is flaky
due to a race between executor shutdown (due to never getting any
tasks) and the test querying resource statistics. If the executor
is shutdown before the statistics query, the test will fail.

This patch fixes the test by explicitly waiting for the task to
be delivered and task status transition to `TASK_RUNNING` before
restarting the agent. This way, the executor will not be shutdown
after agent restart. Hence there will be no race.

Review: https://reviews.apache.org/r/69656

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