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/core/version.t.cpp b/tests/core/version.t.cpp
index 349dfff..2a0e09e 100644
--- a/tests/core/version.t.cpp
+++ b/tests/core/version.t.cpp
@@ -30,8 +30,7 @@
 #include <boost/algorithm/string/classification.hpp>
 #include <boost/algorithm/string/split.hpp>
 
-namespace nfd {
-namespace tests {
+namespace nfd::tests {
 
 BOOST_AUTO_TEST_SUITE(TestVersion)
 
@@ -67,5 +66,4 @@
 
 BOOST_AUTO_TEST_SUITE_END() // TestVersion
 
-} // namespace tests
-} // namespace nfd
+} // namespace nfd::tests