Fixed 'SlaveRecoveryTest.RegisterDisconnectedSlave'.
Fixed 'SlaveRecoveryTest.RegisterDisconnectedSlave'.

The test was broken in 'de11f0ffed36fbabc8fb4167859fd23b75f43f10'
due to the executor sending one more status update.

Additionally, an explicit verification of the status update
acknowledgement was added to eliminate a common source of
test flakyness.

Review: https://reviews.apache.org/r/63177/

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