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.cc b/frc971/control_loops/drivetrain/drivetrain.cc
index 77c52c4..1c0ac4f 100644
--- a/frc971/control_loops/drivetrain/drivetrain.cc
+++ b/frc971/control_loops/drivetrain/drivetrain.cc
@@ -41,6 +41,7 @@
       gyro_reading_fetcher_(
           event_loop->MakeFetcher<::frc971::sensors::GyroReading>(
               "/drivetrain")),
+      down_estimator_(dt_config),
       localizer_(localizer),
       kf_(dt_config_.make_kf_drivetrain_loop()),
       dt_openloop_(dt_config_, &kf_),
@@ -177,6 +178,9 @@
       case IMUType::IMU_Y:
         last_accel_ = -imu_values_fetcher_->accelerometer_y();
         break;
+      case IMUType::IMU_Z:
+        last_accel_ = imu_values_fetcher_->accelerometer_z();
+        break;
     }
   }