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/table/pit-entry.t.cpp b/tests/daemon/table/pit-entry.t.cpp
index 6f97a1c..45076e3 100644
--- a/tests/daemon/table/pit-entry.t.cpp
+++ b/tests/daemon/table/pit-entry.t.cpp
@@ -33,11 +33,9 @@
 
 namespace bdata = boost::unit_test::data;
 
-namespace nfd {
-namespace pit {
-namespace tests {
+namespace nfd::tests {
 
-using namespace nfd::tests;
+using namespace nfd::pit;
 
 BOOST_AUTO_TEST_SUITE(Table)
 BOOST_FIXTURE_TEST_SUITE(TestPitEntry, GlobalIoFixture)
@@ -239,6 +237,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestPitEntry
 BOOST_AUTO_TEST_SUITE_END() // Table
 
-} // namespace tests
-} // namespace pit
-} // namespace nfd
+} // namespace nfd::tests