autotest: fixed MAV_CMD_GUIDED_CHANGE_ALTITUDE

need terrain handlers installed to support terrain targets

Tools/autotest/arduplane.py

@@ -5533,6 +5533,7 @@ class AutoTestPlane(vehicle_test_suite.TestSuite):
 
     def MAV_CMD_GUIDED_CHANGE_ALTITUDE(self):
         '''test handling of MAV_CMD_GUIDED_CHANGE_ALTITUDE'''
+        self.install_terrain_handlers_context()
         self.start_subtest("set home relative altitude")
         self.takeoff(30, relative=True)
         self.change_mode('GUIDED')