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/dummy-validator.hpp b/tests/unit/dummy-validator.hpp
index 63b7556..903fcea 100644
--- a/tests/unit/dummy-validator.hpp
+++ b/tests/unit/dummy-validator.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2022 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).
  *
@@ -26,8 +26,7 @@
 #include "ndn-cxx/security/validation-policy.hpp"
 #include "ndn-cxx/security/certificate-fetcher-offline.hpp"
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 /**
  * \brief A dummy validation policy for unit testing.
@@ -109,7 +108,6 @@
   }
 };
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests
 
 #endif // NDN_CXX_TESTS_UNIT_DUMMY_VALIDATOR_HPP