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/face/udp-channel-fixture.hpp b/tests/daemon/face/udp-channel-fixture.hpp
index ac30738..a314a8c 100644
--- a/tests/daemon/face/udp-channel-fixture.hpp
+++ b/tests/daemon/face/udp-channel-fixture.hpp
@@ -31,9 +31,9 @@
 
 #include "channel-fixture.hpp"
 
-namespace nfd {
-namespace face {
-namespace tests {
+namespace nfd::tests {
+
+using face::UdpChannel;
 
 class UdpChannelFixture : public ChannelFixture<UdpChannel, udp::Endpoint>
 {
@@ -69,8 +69,6 @@
   std::vector<shared_ptr<Face>> clientFaces;
 };
 
-} // namespace tests
-} // namespace face
-} // namespace nfd
+} // namespace nfd::tests
 
 #endif // NFD_TESTS_DAEMON_FACE_UDP_CHANNEL_FIXTURE_HPP