Added min_time and mock time support.

This makes it so we can use monotonic_clock in test cases, and have
a good initialization value.

Change-Id: I1cb8f9773ca5b266042c119878e70b677ef3a45d
diff --git a/aos/common/time.h b/aos/common/time.h
index ba795a9..8f57916 100644
--- a/aos/common/time.h
+++ b/aos/common/time.h
@@ -32,6 +32,9 @@
   }
 
   static constexpr monotonic_clock::duration zero() { return duration(0); }
+
+  static constexpr time_point min_time{
+      time_point(duration(::std::numeric_limits<duration::rep>::min()))};
 };
 
 namespace time {