Added test to verify that frameworks receive OPERATION_DROPPED updates.
Added test to verify that frameworks receive OPERATION_DROPPED updates.

This patch adds the new test
`AgentOperationFeedbackTest.DroppedOperationStatusUpdate`
which verifies that when an operation is dropped on its
way to the agent, a framework will receive an
OPERATION_DROPPED status update if it requested feedback
for that operation.

Review: https://reviews.apache.org/r/69922/

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