…city error is bounded

These changes enable the EKF to use the last observable wind velocity vector estimate to synthesise an airspeed measurement if operating without an airspeed sensor and when all other measurement types that can constrain velocoty drift are lost. This enables the EKF to use dead reckoning to continue after loss of GPS when there is no air speed sensor fitted and without the need to set a default airspeed value.
The logic used to fuse a default airspeed value has also been cleaned up and the call to FuseAirSpeed() from inside SelectBetaDragFusion() has been removed.

AP_NavEKF3: Fix error in default airspeed observation variance

AP_NavEKF3: Enable shadow fusion of airspeed when sensor is disabled

These changes prevent the EKF from consuming GPS data too soon when it is recovering from jamming if the EKF is able to navigate using dead reckoning.

Explicitly test time taken to reset to GPS loss and regain of lock for copter without and plane with dead reckoning assistance.

…ensor

…checks

Make it user selectable.
Remove potential for a race condition between decisions based on latest data and the EKF fusion processing which operates on a delayed time horizon. This is achieved by preventing data entering the buffer if awaiting checks to pass ensuring that no EKF fusion time horizon processes can use data that hasn't passed checks.
Log the waitingForGpsChecks class variable

The previous method resulted in data incest and fusion of predicted airspeed on every EKF internal time step.
This was not apparent during flight where the vehicle was turning, but during long straight legs did not constrain along track drift.

…speed

also minor comment and format fixes