for now, there are no data provider processes, but in the future there
will be one per connection to fuchsia.feedback.DataProvider. This CL is
just to introduce the test utilities.

DX-1497 #comment

TESTED=`fx run-test feedback_agent_tests -t feedback_agent_integration_test`

Change-Id: Ia5cb07334d3d27874318cd6b54fcf4afaa336da0