Fix motors spinning in turtle mode

In the current motortest/turtle path, zero throttle is being mapped to DShot 48 (minimum throttle) rather than DShot 0 (stop). This can be undesirable on low starting and/or more responsive escs (32bits?), causing them to spin unexpectedly. This change adds an explicit MOTOR_OFF mapping for zero throttle values in the motortest_override path so the DShot driver correctly sends stop instead of min throttle. Tested on SP8 and Lucid AIO.