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

Change-Id: I3299e6255e4067b142ad0c8f87bebe1892505d0d
diff --git a/tests/test-certificate-cache.cpp b/tests/test-certificate-cache.cpp
index 29a45fc..3718170 100644
--- a/tests/test-certificate-cache.cpp
+++ b/tests/test-certificate-cache.cpp
@@ -10,8 +10,8 @@
 #include "face.hpp"
 
 using namespace std;
-using namespace ndn;
 
+namespace ndn {
 
 BOOST_AUTO_TEST_SUITE(TestCertificateCache)
 
@@ -55,3 +55,5 @@
 }
 
 BOOST_AUTO_TEST_SUITE_END()
+
+} // namespace ndn