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-config.t.cpp b/tests/unit/security/validation-policy-config.t.cpp
index 83c6f52..07b98e9 100644
--- a/tests/unit/security/validation-policy-config.t.cpp
+++ b/tests/unit/security/validation-policy-config.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).
  *
@@ -30,14 +30,9 @@
 #include "tests/unit/security/validator-config/common.hpp"
 #include "tests/unit/security/validator-fixture.hpp"
 
-namespace ndn {
-namespace security {
-inline namespace v2 {
-namespace validator_config {
-namespace tests {
+namespace ndn::tests {
 
-using namespace ndn::tests;
-using namespace ndn::security::tests;
+using ndn::security::ValidationPolicyConfig;
 
 BOOST_AUTO_TEST_SUITE(Security)
 BOOST_AUTO_TEST_SUITE(TestValidationPolicyConfig)
@@ -760,6 +755,8 @@
 
 BOOST_FIXTURE_TEST_CASE(OrphanedPolicyLoad, HierarchicalValidatorFixture<ValidationPolicyConfig>) // Bug #4758
 {
+  using ndn::security::validator_config::Error;
+
   ValidationPolicyConfig policy1;
   BOOST_CHECK_THROW(policy1.load("trust-anchor { type any }", "test-config"), Error);
 
@@ -767,8 +764,7 @@
   BOOST_CHECK_THROW(policy1.load("trust-anchor { type any }", "test-config"), Error);
 
   ValidationPolicyConfig policy2;
-
-  std::string config = R"CONF(
+  const std::string config = R"CONF(
       trust-anchor
       {
         type dir
@@ -776,7 +772,6 @@
         refresh 1h
       }
     )CONF";
-
   // Inserting trust anchor would have triggered a segfault
   BOOST_CHECK_THROW(policy2.load(config, "test-config"), Error);
 }
@@ -784,8 +779,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestValidationPolicyConfig
 BOOST_AUTO_TEST_SUITE_END() // Security
 
-} // namespace tests
-} // namespace validator_config
-} // inline namespace v2
-} // namespace security
-} // namespace ndn
+} // namespace ndn::tests