Fixed flaky observerTest.TestNotifyAfterDetach

Summary: Not resetting counter matters when running binary directly.

Reviewed By: bwasti

Differential Revision: D5202723

fbshipit-source-id: 7cc5c9e4d5c6db0f79fa3950454556bc26ea4914
diff --git a/caffe2/core/observer_test.cc b/caffe2/core/observer_test.cc
index 44f4ce4..7a086b2 100644
--- a/caffe2/core/observer_test.cc
+++ b/caffe2/core/observer_test.cc
@@ -126,6 +126,7 @@
       make_unique<DummyObserver<SimpleNet>>(
           *(caffe2::dynamic_cast_if_rtti<SimpleNet*>(net.get())));
   net_ob.get()->Deactivate();
+  counter = 0;
   net.get()->Run();
   EXPECT_EQ(0, counter.load());
 }