EV_yaw: reset ev_to_ekf to identity when yaw is reset to EV

[bresch]

Otherwise, the old rotation matrix is used and not updated anymore because the EKF is fusing EV yaw data.

Note that this issue only occurs when MASK_ROTATE_EV and MASK_EV_YAW are set at the same time (because otherwise, if MASK_ROTATE_EV isn't set, the wrong rotation matrix is never used).

Explanation of the race condition:

1. Initially, EV yaw fusion isn't active, the rotation matrix between the local frame (NED, initialized using mag) and EV frame is computed
2. EV yaw starts, the yaw estimate is reset to the EV yaw
3. During the fusion, if MASK_ROTATE_EV is set, the data is rotated using the rotation matrix computed in 1 (which is now wrong)
4. Because EV yaw fusion is now active, the rotation matrix isn't updated anymore

Added a unit test that triggers the race condition above (fails without the fix).

[priseborough]

I've run the test with and without the change to resetYawToEV() and verified it catches the bug.