When the mag is pulled away before takeoff, the EKF can end up learning
a bad yaw gyro bias. This bad gyro bias makes the estimated heading spin
and even if the bias continues to be learned, it can end up being stuck
in limit cycle due to the innovation changing its sign every time the
heading estimate overshoots the measurement.
To prevent this, the Z gyro bias variance is reset when the mag heading
fusion test ratio fails on ground.
bresch
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