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/cs-benchmark.cpp b/tests/other/cs-benchmark.cpp
index fe4eeaa..793ccf1 100644
--- a/tests/other/cs-benchmark.cpp
+++ b/tests/other/cs-benchmark.cpp
@@ -32,8 +32,7 @@
 #include <valgrind/callgrind.h>
 #endif
 
-namespace nfd {
-namespace tests {
+namespace nfd::tests {
 
 class CsBenchmarkFixture
 {
@@ -210,5 +209,4 @@
   std::cout << "find(CanBePrefix-hit) " << (N_INTERESTS * N_CHILDREN * REPEAT) << ": " << d << std::endl;
 }
 
-} // namespace tests
-} // namespace nfd
+} // namespace nfd::tests