tests: refactor common test infrastructure and fixtures

Change-Id: I597c11130eefa2cc2846ee6655c85dc04f2f22ef
diff --git a/tests/unit/security/certificate-fetcher-direct-fetch.t.cpp b/tests/unit/security/certificate-fetcher-direct-fetch.t.cpp
index 25e0be5..7757f5b 100644
--- a/tests/unit/security/certificate-fetcher-direct-fetch.t.cpp
+++ b/tests/unit/security/certificate-fetcher-direct-fetch.t.cpp
@@ -20,9 +20,10 @@
  */
 
 #include "ndn-cxx/security/certificate-fetcher-direct-fetch.hpp"
-#include "ndn-cxx/security/validation-policy-simple-hierarchy.hpp"
+
 #include "ndn-cxx/lp/nack.hpp"
 #include "ndn-cxx/lp/tags.hpp"
+#include "ndn-cxx/security/validation-policy-simple-hierarchy.hpp"
 
 #include "tests/boost-test.hpp"
 #include "tests/unit/security/validator-fixture.hpp"