Convert control loop tests over to simulated event loop

This makes it so that we properly only use ShmEventLoop for running in
realtime on a robot.  Very nice.

Change-Id: I46b770b336f59e08cfaf28511b3bd5689f72fff1
diff --git a/frc971/control_loops/drivetrain/drivetrain.cc b/frc971/control_loops/drivetrain/drivetrain.cc
index b45f016..b9d2bc4 100644
--- a/frc971/control_loops/drivetrain/drivetrain.cc
+++ b/frc971/control_loops/drivetrain/drivetrain.cc
@@ -93,7 +93,8 @@
     const ::frc971::control_loops::DrivetrainQueue::Position *position,
     ::frc971::control_loops::DrivetrainQueue::Output *output,
     ::frc971::control_loops::DrivetrainQueue::Status *status) {
-  monotonic_clock::time_point monotonic_now = monotonic_clock::now();
+  const monotonic_clock::time_point monotonic_now =
+      event_loop()->monotonic_now();
 
   if (!has_been_enabled_ && output) {
     has_been_enabled_ = true;