Fixed flakiness in 'ResourceProviderRegistrarTest.AgentRegistrar'.
Fixed flakiness in 'ResourceProviderRegistrarTest.AgentRegistrar'.

This test created an agent registrar which attempted to store its state
in the agent's meta directory before the agent had set up that
directory hierarchy.

This patch adds additional waiting to make sure required directories
have been set up before starting the agent registrar.

Review: https://reviews.apache.org/r/63024/

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