Refactor and modernize namespace declarations

 * Completely remove inline namespace v2
 * Flatten some unnecessarily deep namespace nesting
 * Move DummyClientFace, Segmenter, SegmentFetcher to namespace ndn
 * Move all unit tests to namespace ndn::tests

Change-Id: I8bcfcf9fd669936a3277d2d5d505f765b4b05742
diff --git a/tests/unit/test-home-env-saver.hpp b/tests/unit/test-home-env-saver.hpp
index 9a04605..db1c2f2 100644
--- a/tests/unit/test-home-env-saver.hpp
+++ b/tests/unit/test-home-env-saver.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2021 Regents of the University of California.
+ * Copyright (c) 2013-2023 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -25,8 +25,7 @@
 #include <cstdlib>
 #include <string>
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 class TestHomeEnvSaver
 {
@@ -50,7 +49,6 @@
   std::string m_HOME;
 };
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests
 
 #endif // NDN_CXX_TESTS_UNIT_TEST_HOME_ENV_SAVER_HPP