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/rib/readvertise/host-to-gateway-readvertise-policy.t.cpp b/tests/daemon/rib/readvertise/host-to-gateway-readvertise-policy.t.cpp
index c3682dd..df73058 100644
--- a/tests/daemon/rib/readvertise/host-to-gateway-readvertise-policy.t.cpp
+++ b/tests/daemon/rib/readvertise/host-to-gateway-readvertise-policy.t.cpp
@@ -29,11 +29,9 @@
 #include "tests/key-chain-fixture.hpp"
 #include "tests/daemon/global-io-fixture.hpp"
 
-namespace nfd {
-namespace rib {
-namespace tests {
+namespace nfd::tests {
 
-using namespace nfd::tests;
+using namespace nfd::rib;
 
 class HostToGatewayReadvertisePolicyFixture : public GlobalIoFixture, public KeyChainFixture
 {
@@ -109,6 +107,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestHostToGatewayReadvertisePolicy
 BOOST_AUTO_TEST_SUITE_END() // Readvertise
 
-} // namespace tests
-} // namespace rib
-} // namespace nfd
+} // namespace nfd::tests