autotest: add tests for mavlink disarm

[peterbarker]

We discussed allowing the operator to disarm via mavlink within the land detector timeout.

We thought on the call it was 10 seconds.

It's actually 4 seconds before the auto-land triggers.

We can perhaps call land_detector with a very low number - but should we?

edit: this makes Plane work like Copter - can't disarm via mavlink mid-air. There's an option bit to get back the old behaviour

[peterbarker]

This PR no longer changes any behavior in the autopilot code, but simply adds a pair of tests to ensure we don't lose the current behavior while not realizing it.