autotest: increase timeout on DO_CHANGE_SPEED test

Tools/autotest/arduplane.py

@@ -692,7 +692,7 @@ class AutoTestPlane(AutoTest):
             self.progress("groundspeed and airspeed should be different (have=%f want=%f)" % (delta, want_delta))
             if delta > want_delta:
                 break
-        self.fly_home_land_and_disarm()
+        self.fly_home_land_and_disarm(timeout=240)
 
     def fly_home_land_and_disarm(self, timeout=120):
         filename = "flaps.txt"