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/containers/priority_queue_test.cc b/aos/containers/priority_queue_test.cc
index 64456c4..f27a206 100644
--- a/aos/containers/priority_queue_test.cc
+++ b/aos/containers/priority_queue_test.cc
@@ -2,8 +2,7 @@
 
 #include "gtest/gtest.h"
 
-namespace aos {
-namespace testing {
+namespace aos::testing {
 
 // Effectively copies the implementation of ::std::less just to demonstrate how
 // things work.
@@ -172,5 +171,4 @@
   EXPECT_EQ(11, it->a);
 }
 
-}  // namespace testing
-}  // namespace aos
+}  // namespace aos::testing