Update the unit test stubs as per API changes

Author: Deepika

UNITTESTS/stubs/EventQueue_stub.cpp

@@ -60,7 +60,7 @@ void EventQueue::background(Callback<void(int)> update)
 {
 }
 
-void EventQueue::chain(EventQueue *target)
+int EventQueue::chain(EventQueue *target)
 {
 }
 

UNITTESTS/stubs/equeue_stub.c

@@ -103,7 +103,7 @@ void equeue_background(equeue_t *queue,
 
 }
 
-void equeue_chain(equeue_t *queue, equeue_t *target)
+int equeue_chain(equeue_t *queue, equeue_t *target)
 {
 
 }