Future-proofed some slave removal tests.
Future-proofed some slave removal tests.

These tests relied on the implementation detail that when an agent is
removed from the list of registered agents, the master sends a
ShutdownSlaveMessage to the agent. That will change in the future
(MESOS-4049). To prepare for this future planned behavior, adjust these
tests to be more robust by instead checking for the invocation of the
`slaveLost` scheduler callback.

Review: https://reviews.apache.org/r/50422/

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