Add test for EK3_ORGN_HGT_MASK

[peterbarker]

• accumulate simulated barometer drift rather than setting based on vehicle uptime
• add description for SIM_BARO_DRIFT
• bugfixes for hook removal in exception handling

... and, of course, add a test that we adjust the barometer altitude over time.If the EKF parameter is not set then the test doesn't pass. In fact, it fails with a flow-of-control error in the landing detector....

This work sponsored by Harris Aerial

[peterbarker]