util: add literal operators for duration types

Change-Id: I007fa61c80e535c6e4b12e85a32eeeee62fecc3c
Refs: #4468
diff --git a/tests/identity-management-fixture.cpp b/tests/identity-management-fixture.cpp
index ee80dd1..79d5eb2 100644
--- a/tests/identity-management-fixture.cpp
+++ b/tests/identity-management-fixture.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -87,8 +87,8 @@
   request.setName(request.getKeyName().append("parent").appendVersion());
 
   SignatureInfo info;
-  info.setValidityPeriod(security::ValidityPeriod(time::system_clock::now(),
-                                                  time::system_clock::now() + time::days(7300)));
+  auto now = time::system_clock::now();
+  info.setValidityPeriod(security::ValidityPeriod(now, now + 7300_days));
 
   v2::AdditionalDescription description;
   description.set("type", "sub-certificate");
@@ -112,15 +112,15 @@
 
   // set metainfo
   certificate.setContentType(tlv::ContentType_Key);
-  certificate.setFreshnessPeriod(time::hours(1));
+  certificate.setFreshnessPeriod(1_h);
 
   // set content
   certificate.setContent(key.getPublicKey().data(), key.getPublicKey().size());
 
   // set signature-info
   SignatureInfo info;
-  info.setValidityPeriod(security::ValidityPeriod(time::system_clock::now(),
-                                                  time::system_clock::now() + time::days(10)));
+  auto now = time::system_clock::now();
+  info.setValidityPeriod(security::ValidityPeriod(now, now + 10_days));
 
   m_keyChain.sign(certificate, signingByKey(key).setSignatureInfo(info));
   return certificate;