Nest some more namespaces

Did someone previously suggest that all namespace had been
nested?  Silly.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I22278c1caaeba8b47dc46fb2ed3078c20a11e190
diff --git a/aos/events/message_counter.h b/aos/events/message_counter.h
index 418c1eb..13d75c5 100644
--- a/aos/events/message_counter.h
+++ b/aos/events/message_counter.h
@@ -3,8 +3,7 @@
 
 #include "aos/events/event_loop.h"
 
-namespace aos {
-namespace testing {
+namespace aos::testing {
 
 // Simple class to count messages on a channel easily.  This only counts
 // messages published while running.
@@ -22,7 +21,6 @@
   size_t count_ = 0;
 };
 
-}  // namespace testing
-}  // namespace aos
+}  // namespace aos::testing
 
 #endif  // AOS_EVENTS_MESSAGE_COUNTER_H_