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

Change-Id: I3299e6255e4067b142ad0c8f87bebe1892505d0d
diff --git a/tests/test-faces.cpp b/tests/test-faces.cpp
index a8d6135..67c5add 100644
--- a/tests/test-faces.cpp
+++ b/tests/test-faces.cpp
@@ -8,7 +8,7 @@
 #include "face.hpp"
 
 using namespace std;
-using namespace ndn;
+namespace ndn {
 
 BOOST_AUTO_TEST_SUITE(TestFaces)
 
@@ -93,3 +93,5 @@
 }
 
 BOOST_AUTO_TEST_SUITE_END()
+
+} // namespace ndn