autotest: add a test for switching to auto with low speed and alt

[peterbarker]

SITL/autotest reproduction of something which happened in a real flight.

After switching to auto, TECS doesn't seem to get updated, throttle goes to zero, vehicle flies into ground (or close to it, in the real flight).

Note the messages includes "holding course", but never "Takeoff level-off starting at". That's because relative_alt_cm is below 1000cm. That's unlikely to change if our throttle is zero and we're below that value to start with.

We are suppressing throttle because we are below 5m-rel-alt:

[tridge]

@peterbarker ok, so it is the throttle suppression heuristics?

[peterbarker]

@peterbarker ok, so it is the throttle suppression heuristics?

Yes, it appeared so in my debugging session.

I modified the test:

diff --git a/Tools/autotest/arduplane.py b/Tools/autotest/arduplane.py
index d8b22a8383f..8b94c218385 100644
--- a/Tools/autotest/arduplane.py
+++ b/Tools/autotest/arduplane.py
@@ -5447,6 +5447,8 @@ class AutoTestPlane(vehicle_test_suite.TestSuite):
         self.takeoff_in_FBWA(3, throttle=1450)
         self.set_rc(3, 1200)
         self.change_mode('AUTO')
+        self.delay_sim_time(0.5)
+        self.send_debug_trap()
         self.wait_current_waypoint(3)
         self.fly_home_land_and_disarm()

and ran the simulation

./Tools/autotest/autotest.py --gdb --debug test.Plane.AutoTakeoffWhileFlying  --disable-break -B Plane::suppress_throttle

... then in gdb

ena
c

to start walking through the relevant code.

[tridge]

we've decided a FLIGHT_OPTIONS bit to disable the suppression code would be worthwhile