Refactor unit-testing of core/service_impl.cc

This just improves unit-testing of ServiceImpl by
introducing MockProducer/ MockConsumer classes.
Also this renames (without adding any new behavior)
the existing methods Producer::OnTracing{Start,StopTracing}
as follows:
- OnTracingStop -> removed as it's currently unsupported
  (see b/77532839)
- OnTracingStart -> renamed to OnTracingSetup, because
  this is what it does. Also this name conflicts with
  Consumer::OnTracingStart, which has a different semantic
  and happens at different times.

Also this CL renames Consumer::OnTracingStop to
OnTracingDisabled to match the {Enable,Disable}Tracing methods.

Change-Id: Ided455d3b37cfefdfbc3eda94e5feccaeeb15a5d
diff --git a/test/test_helper.h b/test/test_helper.h
index b63707c..563345f 100644
--- a/test/test_helper.h
+++ b/test/test_helper.h
@@ -36,7 +36,7 @@
   // Consumer implementation.
   void OnConnect() override;
   void OnDisconnect() override;
-  void OnTracingStop() override;
+  void OnTracingDisabled() override;
   void OnTraceData(std::vector<TracePacket> packets, bool has_more) override;
 
   void StartServiceIfRequired();