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/key-chain-fixture.cpp b/tests/key-chain-fixture.cpp
index 7bbcf6a..fefc7cf 100644
--- a/tests/key-chain-fixture.cpp
+++ b/tests/key-chain-fixture.cpp
@@ -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).
  *
@@ -25,8 +25,7 @@
 
 #include <boost/filesystem/operations.hpp>
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 using namespace ndn::security;
 
@@ -91,5 +90,4 @@
   return saveIdentityCert(id, filename);
 }
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests