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/util/file_test.cc b/aos/util/file_test.cc
index 905a2cd..b3f1e66 100644
--- a/aos/util/file_test.cc
+++ b/aos/util/file_test.cc
@@ -10,9 +10,7 @@
 #include "aos/realtime.h"
 #include "aos/testing/tmpdir.h"
 
-namespace aos {
-namespace util {
-namespace testing {
+namespace aos::util::testing {
 
 using ::testing::ElementsAre;
 
@@ -144,6 +142,4 @@
   EXPECT_EQ("", ReadFileToStringOrDie(test_file));
 }
 
-}  // namespace testing
-}  // namespace util
-}  // namespace aos
+}  // namespace aos::util::testing