Fix test code for MockWdmSubscriptionResponder

The test code for managing the binding in the
MockWdmSubscriptionResponder.  The logic bug was present for some
time, but skipped past unnoticed.  The core issue is as follows: we
grab a reference to `gBinding` when the SubscriptionEngine generates a
`kEvent_OnSubscriptionEstablished` (line 654 above the patch); that
reference is logically bound to the publisher and no to the client
(and to the mutual countersubscription).  As a result, the reference
to the binding object will be released as a result of
SubscriptionEngine generating a `kEvent_OnSubscriptionTerminated`; in
this code this takes place as a result of calling
`HandlePublisherRelease()` in line 802 below.  The error handling
takes place via checking for errors in the `exit:` label and takes a
form of `EndSubscription()`. As a result of both of these properties,
It is a logical error to `Release()` the binding because of a failed
call `NewClient()`.

Author: robszewczyk

src/test-apps/MockWdmSubscriptionResponder.cpp

@@ -737,7 +737,7 @@ void MockWdmSubscriptionResponderImpl::PublisherEventCallback (void * const aApp
                     aInParam.mSubscriptionEstablished.mHandler->GetBinding(),
                     responder, ClientEventCallback,
                     &(responder->mSinkCatalog), kResponseTimeoutMsec * 2);
-                VerifyOrExit(err == WEAVE_NO_ERROR, gBinding->Release());
+                SuccessOrExit(err);
 
                 // TODO: EVENT-DEMO
                 responder->mSubscriptionClient->InitiateCounterSubscription(