Avoided using SIGUSR1 in two test cases.
Avoided using SIGUSR1 in two test cases.

We want to cause the agent to shutdown gracefully (i.e., to send an
"unregister" message to the master). This can be accomplished by sending
the whole process a SIGUSR1 but that seems fragile; using the agent's
`shutdown()` method seems more robust.

Review: https://reviews.apache.org/r/52039/

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