shill: usage of EventDispatcherForTest and NullRPCIdentifier

Switch tests to use EventDispatcherForTest instead of EventDispatcher,
since EventDispatcher no longer owns a message loop.

Update references for DBusAdaptor::kNullPath to
ControlInterface::NullRPCIdentifier, since RPC identifier is specific
to control interface and opaque to the internal code.

BUG=chromium:517680
TEST=unittests

Change-Id: Ia932143e433b44c84bd176baac660d1fa75261d8
Reviewed-on: https://chromium-review.googlesource.com/293550
Commit-Ready: Zeping Qiu <[email protected]>
Tested-by: Zeping Qiu <[email protected]>
Reviewed-by: Paul Stewart <[email protected]>
diff --git a/result_aggregator_unittest.cc b/result_aggregator_unittest.cc
index ec88daa..3ecf114 100644
--- a/result_aggregator_unittest.cc
+++ b/result_aggregator_unittest.cc
@@ -10,6 +10,7 @@
 #include <gtest/gtest.h>
 
 #include "shill/mock_event_dispatcher.h"
+#include "shill/test_event_dispatcher.h"
 #include "shill/testing.h"
 
 namespace shill {
@@ -54,7 +55,7 @@
   }
 
  protected:
-  EventDispatcher dispatcher_;
+  EventDispatcherForTest dispatcher_;
 };
 
 class ResultAggregatorTestWithMockDispatcher : public ResultAggregatorTest {