Fixed flakyness in MasterTests.RegistryUpdateAfterReconfiguration.
Fixed flakyness in MasterTests.RegistryUpdateAfterReconfiguration.

It could happen that an agent resends its `RegisterSlaveMessage`
and with the correct timing, the master would assume that it
comes from a new agent with the same pid and store its information
in the registry. See MESOS-8336.

Review: https://reviews.apache.org/r/65758/

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