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/tools/nfdc/help.t.cpp b/tests/tools/nfdc/help.t.cpp
index 6f5392c..f629f62 100644
--- a/tests/tools/nfdc/help.t.cpp
+++ b/tests/tools/nfdc/help.t.cpp
@@ -29,10 +29,7 @@
 
 #include <boost/test/tools/output_test_stream.hpp>
 
-namespace nfd {
-namespace tools {
-namespace nfdc {
-namespace tests {
+namespace nfd::tools::nfdc::tests {
 
 BOOST_AUTO_TEST_SUITE(Nfdc)
 BOOST_AUTO_TEST_SUITE(TestHelp)
@@ -100,7 +97,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestCommandParser
 BOOST_AUTO_TEST_SUITE_END() // Nfdc
 
-} // namespace tests
-} // namespace nfdc
-} // namespace tools
-} // namespace nfd
+} // namespace nfd::tools::nfdc::tests