tests: Do not use ``using namespace ndn'' and defining all tests in ndn namespace

Change-Id: I3299e6255e4067b142ad0c8f87bebe1892505d0d
diff --git a/tests/test-sec-public-info-sqlite3.cpp b/tests/test-sec-public-info-sqlite3.cpp
index 233109c..6e684e9 100644
--- a/tests/test-sec-public-info-sqlite3.cpp
+++ b/tests/test-sec-public-info-sqlite3.cpp
@@ -13,8 +13,7 @@
 #include "security/key-chain.hpp"
 
 using namespace std;
-using namespace ndn;
-
+namespace ndn {
 
 BOOST_AUTO_TEST_SUITE(TestSecPublicInfoSqlite3)
 
@@ -73,3 +72,5 @@
 }
 
 BOOST_AUTO_TEST_SUITE_END()
+
+} // namespace ndn