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/fw/congestion-mark-strategy.cpp b/tests/other/fw/congestion-mark-strategy.cpp
index 3d920c7..078799b 100644
--- a/tests/other/fw/congestion-mark-strategy.cpp
+++ b/tests/other/fw/congestion-mark-strategy.cpp
@@ -25,8 +25,7 @@
 
 #include "congestion-mark-strategy.hpp"
 
-namespace nfd {
-namespace fw {
+namespace nfd::fw {
 
 NFD_REGISTER_STRATEGY(CongestionMarkStrategy);
 
@@ -91,5 +90,4 @@
   }
 }
 
-} // namespace fw
-} // namespace nfd
+} // namespace nfd::fw