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/security/validation-policy-signed-interest.t.cpp b/tests/unit/security/validation-policy-signed-interest.t.cpp
index 58f502e..ded8825 100644
--- a/tests/unit/security/validation-policy-signed-interest.t.cpp
+++ b/tests/unit/security/validation-policy-signed-interest.t.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).
  *
@@ -30,12 +30,9 @@
 
 #include <boost/mpl/vector.hpp>
 
-namespace ndn {
-namespace security {
-inline namespace v2 {
-namespace tests {
+namespace ndn::tests {
 
-using namespace ndn::tests;
+using namespace ndn::security;
 
 BOOST_AUTO_TEST_SUITE(Security)
 
@@ -525,7 +522,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestValidationPolicySignedInterest
 BOOST_AUTO_TEST_SUITE_END() // Security
 
-} // namespace tests
-} // inline namespace v2
-} // namespace security
-} // namespace ndn
+} // namespace ndn::tests