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/test-home-fixture.hpp b/tests/test-home-fixture.hpp
index 5778a6e..9801c92 100644
--- a/tests/test-home-fixture.hpp
+++ b/tests/test-home-fixture.hpp
@@ -32,8 +32,7 @@
 #include <boost/filesystem/operations.hpp>
 #include <boost/filesystem/path.hpp>
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 /**
  * @brief Fixture to adjust/restore NDN_CLIENT_PIB and NDN_CLIENT_TPM paths
@@ -121,7 +120,6 @@
   static constexpr std::string_view PATH{"build/keys"};
 };
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests
 
 #endif // NDN_CXX_TESTS_TEST_HOME_FIXTURE_HPP