[feedback] add utilies to check the number of data provider processes in tests

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