Updated `SlaveRecoveryTest.RecoverCompletedExecutor` to verify gc.
Updated `SlaveRecoveryTest.RecoverCompletedExecutor` to verify gc.

In the test `SlaveRecoveryTest.RecoverCompletedExecutor`, when the
completed executor is recovered, verify its work and meta directories
gc'ed successfully.

Review: https://reviews.apache.org/r/65263

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