EKF3: fix EKF reported altitude against public origin

[tridge]

This fixes an issue that happens with the following scenario:

• EK3_AFFINITY=1, giving GPS affinity
• EK3_SRC1_POSZ=3 using GPS altitude
• dual GPS enabled
• at startup the two GPS modules give different AMSL alt
• this causes XKF1[0].OH and XKF1[1].OH to differ by the GPS height difference
• the difference in AMSL alt between the GPS modules goes away before takeoff
• this causes XKF1.PD to compensate for the difference in origin
• takeoff copter in GUIDED mode
• we get a temporary glitch that causes a lane change
• on the new lane the reported above-origin alt changes, causing copter to change altitude (in the provided log it led to a crash)

This PR adds a test that reproduces the issue in SITL and also a fix so that the reported origin uses the public origin and getPosD() in EKF3 adjusts for the difference in origin alt between public origin and the local lane origin

[rmackay9]

I guess we can't remove each core's local origin completely? I suspect this isn't possible because the EKF may update the origin in some rarely used code paths.. but I'd be tempted to remove those code paths (assuming they exist) as well.

[tridge]

I guess we can't remove each core's local origin completely?

no, we can't remove those. The EKF constantly moves the local origin to be the current position while flying so that the locally flat-earth assumption is maintained. The public origin doesn't move. This local origin separation is what allows us to fly long distances (say over 1000km) without large errors due to the curvature of the earth