autotest: added AltResetBadGPS test

this tests the bug in handling a glitching GPS with low accuracy with
AHRS alt reset

Tools/autotest/arduplane.py

@@ -4145,6 +4145,38 @@ class AutoTestPlane(AutoTest):
             0) # button mask
         self.fly_home_land_and_disarm()
 
+    def AltResetBadGPS(self):
+        '''Tests the handling of poor GPS lock pre-arm alt resets'''
+        self.set_parameters({
+            "SIM_GPS_GLITCH_Z": 0,
+            "SIM_GPS_ACC": 0.3,
+        })
+        self.wait_ready_to_arm()
+
+        m = self.assert_receive_message('GLOBAL_POSITION_INT')
+        relalt = m.relative_alt*0.001
+        if abs(relalt) > 3:
+            raise NotAchievedException("Bad relative alt %.1f" % relalt)
+
+        self.progress("Setting low accuracy, glitching GPS")
+        self.set_parameter("SIM_GPS_ACC", 40)
+        self.set_parameter("SIM_GPS_GLITCH_Z", -47)
+
+        self.progress("Waiting 10s for height update")
+        self.delay_sim_time(10)
+
+        self.wait_ready_to_arm()
+        self.arm_vehicle()
+
+        m = self.assert_receive_message('GLOBAL_POSITION_INT')
+        relalt = m.relative_alt*0.001
+        if abs(relalt) > 3:
+            raise NotAchievedException("Bad glitching relative alt %.1f" % relalt)
+
+        self.disarm_vehicle()
+        # reboot to clear potentially bad state
+        self.reboot_sitl()
+
     def tests(self):
         '''return list of all tests'''
         ret = super(AutoTestPlane, self).tests()
@@ -4225,6 +4257,7 @@ class AutoTestPlane(AutoTest):
             self.AerobaticsScripting,
             self.MANUAL_CONTROL,
             self.WindEstimates,
+            self.AltResetBadGPS,
         ])
         return ret