Nest all namespaces

The compiler supports this now.  We can type less going forward.
No functional changes.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I29d6fa4f9aacc0e381f1a7637294db0392466995
diff --git a/aos/events/pingpong_test.cc b/aos/events/pingpong_test.cc
index d35192a..c52e8eb 100644
--- a/aos/events/pingpong_test.cc
+++ b/aos/events/pingpong_test.cc
@@ -7,8 +7,7 @@
 #include "aos/json_to_flatbuffer.h"
 #include "aos/testing/path.h"
 
-namespace aos {
-namespace testing {
+namespace aos::testing {
 
 using aos::testing::ArtifactPath;
 
@@ -168,5 +167,4 @@
   EXPECT_EQ(pi2_ping_count, 1001);
 }
 
-}  // namespace testing
-}  // namespace aos
+}  // namespace aos::testing