test(vehicle): add -reversetest and record the reverse is wheel-spin, not throttle

-reversetest holds negative throttle; forward throttle drives -224.9 km/h and reverse
throttle -99.5 km/h, both backward, so the throttle sign is not the cause - the wheel
spin is inverted relative to the engine torque. Mesh orientation is fine (nose +X).
Next: check the imported wheel-bone reference pose (axle may land on X/Z, not Y).

Game/GigLine/Source/GigLine/GigLineVehicle.cpp

@@ -318,7 +318,8 @@ void AGigLineVehicle::Tick(float DeltaSeconds)
 		{
 			if (DriveElapsed > 1.0f && DriveElapsed < 4.0f)
 			{
-				Movement->SetThrottleInput(1.0f);
+				const float Throttle = FParse::Param(FCommandLine::Get(), TEXT("reversetest")) ? -1.0f : 1.0f;
+				Movement->SetThrottleInput(Throttle);
 			}
 			else if (DriveElapsed >= 4.0f && !bDriveReported)
 			{