Refactor and modernize namespace declarations

Move all unit tests to namespace nfd::tests

Delete unused header core/algorithm.hpp

Change-Id: I5591f0c5f3bb5db67f8b45fae95471f8a555ca68
diff --git a/tests/other/face-benchmark.cpp b/tests/other/face-benchmark.cpp
index 0b809c6..c9a28b6 100644
--- a/tests/other/face-benchmark.cpp
+++ b/tests/other/face-benchmark.cpp
@@ -37,8 +37,7 @@
 #include <valgrind/callgrind.h>
 #endif
 
-namespace nfd {
-namespace tests {
+namespace nfd::tests {
 
 class FaceBenchmark
 {
@@ -171,8 +170,7 @@
   std::vector<std::pair<FaceUri, FaceUri>> m_faceUris;
 };
 
-} // namespace tests
-} // namespace nfd
+} // namespace nfd::tests
 
 int
 main(int argc, char** argv)