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/y2020/vision/sift/fast_gaussian_test.cc b/y2020/vision/sift/fast_gaussian_test.cc
index 0ed9e20..27bab54 100644
--- a/y2020/vision/sift/fast_gaussian_test.cc
+++ b/y2020/vision/sift/fast_gaussian_test.cc
@@ -5,9 +5,7 @@
 
 #include "y2020/vision/sift/fast_gaussian_all.h"
 
-namespace frc971 {
-namespace vision {
-namespace testing {
+namespace frc971::vision::testing {
 
 class FastGaussianTest : public ::testing::Test {
  public:
@@ -70,6 +68,4 @@
   ExpectEqual(fast, fast_direct, 0);
 }
 
-}  // namespace testing
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision::testing