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/flatbuffer_merge_test.cc b/aos/flatbuffer_merge_test.cc
index 4c59879..a2ac088 100644
--- a/aos/flatbuffer_merge_test.cc
+++ b/aos/flatbuffer_merge_test.cc
@@ -9,8 +9,7 @@
 #include "aos/json_to_flatbuffer.h"
 #include "aos/json_to_flatbuffer_generated.h"
 
-namespace aos {
-namespace testing {
+namespace aos::testing {
 
 std::string_view FromFbb(const flatbuffers::FlatBufferBuilder &fbb) {
   return std::string_view(
@@ -589,5 +588,4 @@
 // TODO(austin): unions
 // TODO(austin): struct
 
-}  // namespace testing
-}  // namespace aos
+}  // namespace aos::testing