autotest: Plane.TerrainRally is broken

## Bug Report

### Issue details

Plane.TerrainRally doesn't really test anything right now, due to some alt frame issues. rally_loc and guided_loc are `mavutil.location` objects, correctly populated with AMSL altitudes, but they are being sent as relative alt and terrain alt over the wire. As a result, we're climbing away like crazy and merely asserting it remains above 50m AGL at all times.

I want to fix some stuff with the rally upload helper, and an unrelated bug in WaitAndMaintainLocation, but those will both lead to some side effects on this test. I'd rather disable it in the meantime and come back to fix it.

Start at the Plane.TerrainRally autotest named in the issue and inspect how it is enabled alongside neighboring Plane tests. Temporarily disable this test, then run the relevant Plane autotest selection to confirm TerrainRally is skipped without affecting the other tests.

Written by the indexing model from the issue text.