Make drivetrain_test_lib handle IMU transforms

This also updates the 2020 imu transform to better match the 2020
drivetrain. I haven't actually checked the yaw orientation of the IMU
though.

Change-Id: Ib036705269dfc467a60714c456751a6cfca8a67e
diff --git a/frc971/control_loops/drivetrain/drivetrain_config.h b/frc971/control_loops/drivetrain/drivetrain_config.h
index 5a29008..a33ff22 100644
--- a/frc971/control_loops/drivetrain/drivetrain_config.h
+++ b/frc971/control_loops/drivetrain/drivetrain_config.h
@@ -37,6 +37,7 @@
   IMU_X = 0,          // Use the x-axis of the IMU.
   IMU_Y = 1,          // Use the y-axis of the IMU.
   IMU_FLIPPED_X = 2,  // Use the flipped x-axis of the IMU.
+  IMU_Z = 3,          // Use the z-axis of the IMU.
 };
 
 template <typename Scalar = double>