unittests: Test_LoRaWANTimer.start: Fix test failure

We weren't initialising the mocks correctly, so an MBED_ASSERT failed in
`LoRaWANTimer::start` because the "timer_id" was not set.

Author: rwalton-arm

connectivity/lorawan/tests/UNITTESTS/features/lorawan/lorawantimer/Test_LoRaWANTimer.cpp

@@ -71,9 +71,12 @@ TEST_F(Test_LoRaWANTimer, init)
 
 TEST_F(Test_LoRaWANTimer, start)
 {
-    equeue_stub.void_ptr = NULL;
+    struct equeue_event ptr;
+    equeue_stub.void_ptr = &ptr;
     timer_event_t ev;
     memset(&ev, 0, sizeof(ev));
+    object->init(ev, my_callback);
+    equeue_stub.call_cb_immediately = true;
     object->start(ev, 10);
 }