unit-tests: Make sure to flush the IKE_SA manager before destroying the sender

As the static plugin that creates and destroys the default sender was
not initialized because of the missing socket the daemon won't destroy
our sender.  Test cases will eventually have to flush the IKE_SA manager to
satisfy the leak detective.  However, in case of a test failure and if there
are IKE_SAs in the manager the daemon will flush the SAs when deinitializing,
which will cause deletes to get sent.  This crashes if the sender is already
destroyed.

src/libcharon/tests/utils/exchange_test_helper.c

@@ -354,7 +354,9 @@ void exchange_test_helper_deinit()
 	}
 	lib->credmgr->remove_set(lib->credmgr, &this->creds->set);
 	this->creds->destroy(this->creds);
-	/* can't let charon do it as it happens too late */
+	/* flush SAs before destroying the sender (in case of test failures) */
+	charon->ike_sa_manager->flush(charon->ike_sa_manager);
+	/* charon won't destroy this as it didn't initialize the original sender */
 	charon->sender->destroy(charon->sender);
 	charon->sender = NULL;
 	array_destroy(this->listeners);