tests: avoid deprecated boost headers

Change-Id: I05aa8d5fd22843415db09478f7b53f1810b38db3
diff --git a/tests/unit/util/backports.t.cpp b/tests/unit/util/backports.t.cpp
index 8f80263..b73465b 100644
--- a/tests/unit/util/backports.t.cpp
+++ b/tests/unit/util/backports.t.cpp
@@ -25,7 +25,12 @@
 #include "tests/boost-test.hpp"
 
 #include <numeric>
+
+#if BOOST_VERSION >= 105900
+#include <boost/test/tools/output_test_stream.hpp>
+#else
 #include <boost/test/output_test_stream.hpp>
+#endif
 
 namespace ndn {
 namespace tests {