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/limited-io.cpp b/tests/daemon/limited-io.cpp
index c7e9b1b..9f10f99 100644
--- a/tests/daemon/limited-io.cpp
+++ b/tests/daemon/limited-io.cpp
@@ -29,8 +29,7 @@
 
 #include <boost/exception/diagnostic_information.hpp>
 
-namespace nfd {
-namespace tests {
+namespace nfd::tests {
 
 LimitedIo::LimitedIo(GlobalIoTimeFixture* fixture)
   : m_fixture(fixture)
@@ -107,5 +106,4 @@
   }
 }
 
-} // namespace tests
-} // namespace nfd
+} // namespace nfd::tests