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-common.cpp b/tests/test-common.cpp
index b0d06ea..e6c726e 100644
--- a/tests/test-common.cpp
+++ b/tests/test-common.cpp
@@ -21,8 +21,7 @@
 
 #include "tests/test-common.hpp"
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 shared_ptr<Interest>
 makeInterest(const Name& name, bool canBePrefix, std::optional<time::milliseconds> lifetime,
@@ -61,5 +60,4 @@
   return nack;
 }
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests