Copter: auto takeoff may trigger terrain failsafe

[rmackay9]

This resolves issue #21488 which can lead to a "climb away" in Auto during the Takeoff command with Frame = Terrain.

Below is a screenshot of a SITL test confirming that with the setup mentioned in the issue, the terrain failsafe is correctly triggered and the vehicle RTLs home.

Additional testing we should do includes:

• Confirm the behaviour when the terrain failsafe triggers while the vehicle is landed (it should just disarm) -- auto_takeoff_start silently falls back to alt-above-current-alt
• Test the payload place command that may also run takeoff_run() -- payload place never uses takeoff with terrain following
• Test in Guided mode which may also run auto_takeoff_run() although it doesn't seem to have the same bug -- GUIDED mode has a special check to catch exactly this problem (see code here) although it is still possible to trigger the problem by reducing RNGFND1_MAX_CM after takeoff (e.g. purposely fail the rangefinder). The fix in this PR resolves this very rare failure in Guided mode as well.

[peterbarker]

This looks like the right fix.

However, I am unable to reproduce the problem as the mission-terrain-prearm check does not allow me to arm the Copter.

[peterbarker]

I've created an autotest which tries to reproduce the bug and fails - it dies with a prearm check, as it should

[rmackay9]

@peterbarker,

Great, thanks for the approval.

I've conversely almost never seen the pre-arm failure you mention. I wonder if in your tests you've been disabling the terrain database instead of the rangefinder? I've put the exact steps in issue #21488

Anyway, txs again.