[wpilib] Add TimedRobot unit tests (#2771)

To make the tests reliable, the synchronization in simulation Notifiers
had to be reworked. StepTiming() now waits for all Notifiers to reach
HAL_WaitForNotifierAlarm(), then steps the time, then lets any expired
Notifiers run.

While there, we made some variable names more descriptive and added more
comments.

hal/src/main/native/sim/MockHooks.cpp

@@ -92,6 +92,7 @@ void HALSIM_ResumeTiming(void) {
 HAL_Bool HALSIM_IsTimingPaused(void) { return IsTimingPaused(); }
 
 void HALSIM_StepTiming(uint64_t delta) {
+  WaitNotifiers();
   StepTiming(delta);
   WakeupWaitNotifiers();
 }