Ensured that MasterAPITest.ReservationUpdate waits for changes on agent.
Ensured that MasterAPITest.ReservationUpdate waits for changes on agent.

This patch fixes flakiness in this test by making sure that,
before checking agent state via agent API, not only RESERVE_RESOURCES
call of master V1 API returns, but also the agent completes processing
messages from master.

Review: https://reviews.apache.org/r/72208

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