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/y2019/control_loops/drivetrain/localizer_test.cc b/y2019/control_loops/drivetrain/localizer_test.cc
index 28f0235..7f0b13d 100644
--- a/y2019/control_loops/drivetrain/localizer_test.cc
+++ b/y2019/control_loops/drivetrain/localizer_test.cc
@@ -19,9 +19,7 @@
 
 DEFINE_bool(plot, false, "If true, plot");
 
-namespace y2019 {
-namespace control_loops {
-namespace testing {
+namespace y2019::control_loops::testing {
 
 using ::y2019::constants::Field;
 
@@ -694,6 +692,4 @@
             /*goal_tolerance=*/0.7,
         })));
 
-}  // namespace testing
-}  // namespace control_loops
-}  // namespace y2019
+}  // namespace y2019::control_loops::testing