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/die_test.cc b/aos/die_test.cc
index 8c0a703..6b33b50 100644
--- a/aos/die_test.cc
+++ b/aos/die_test.cc
@@ -2,13 +2,11 @@
 
 #include "gtest/gtest.h"
 
-namespace aos {
-namespace testing {
+namespace aos::testing {
 
 TEST(DieDeathTest, Works) {
   EXPECT_EXIT(Die("str=%s num=%d\n", "hi", 5),
               ::testing::KilledBySignal(SIGABRT), ".*str=hi num=5\n");
 }
 
-}  // namespace testing
-}  // namespace aos
+}  // namespace aos::testing