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/daemon/fw/strategy-tester.hpp b/tests/daemon/fw/strategy-tester.hpp
index 5391dde..e8e795a 100644
--- a/tests/daemon/fw/strategy-tester.hpp
+++ b/tests/daemon/fw/strategy-tester.hpp
@@ -30,11 +30,7 @@
 
 #include "tests/daemon/limited-io.hpp"
 
-namespace nfd {
-namespace fw {
-namespace tests {
-
-using namespace nfd::tests;
+namespace nfd::tests {
 
 /** \brief Extends strategy S for unit testing.
  *
@@ -154,8 +150,6 @@
   std::vector<SendNackArgs> sendNackHistory;
 };
 
-} // namespace tests
-} // namespace fw
-} // namespace nfd
+} // namespace nfd::tests
 
 #endif // NFD_TESTS_DAEMON_FW_STRATEGY_TESTER_HPP