rib: change FIB update mocking method

Previously, FIB update mocking occurs in Rib class. Test coverage
for FibUpdater class is limited to update computation only.
Now, mocking is moved to a subclass of FibUpdater. Test coverage
for FibUpdater class extends to include FIB update queuing.

Previously, mocked FIB update succeeds synchronously.
Now, mocked FIB update completes via io.post, which better
reflects reality that FIB commands are asynchronous.

refs #4731

Change-Id: I563edda5d398eb915f22fecd4b987f32c610d323
diff --git a/tests/daemon/global-io-fixture.hpp b/tests/daemon/global-io-fixture.hpp
index 6dbc891..544c466 100644
--- a/tests/daemon/global-io-fixture.hpp
+++ b/tests/daemon/global-io-fixture.hpp
@@ -43,6 +43,11 @@
 
   ~GlobalIoFixture();
 
+  /** \brief Poll the global io_service.
+   */
+  size_t
+  pollIo();
+
 protected:
   /** \brief Reference to the global io_service instance.
    */