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

Change-Id: I3299e6255e4067b142ad0c8f87bebe1892505d0d
diff --git a/tests/test-encode-decode-interest.cpp b/tests/test-encode-decode-interest.cpp
index 6c2a40e..db193e8 100644
--- a/tests/test-encode-decode-interest.cpp
+++ b/tests/test-encode-decode-interest.cpp
@@ -9,7 +9,7 @@
 #include "interest.hpp"
 
 using namespace std;
-using namespace ndn;
+namespace ndn {
 
 BOOST_AUTO_TEST_SUITE(TestInterest)
 
@@ -79,3 +79,5 @@
 }
 
 BOOST_AUTO_TEST_SUITE_END()
+
+} // namespace ndn