build: migrate to C++17

Change-Id: Ic66b1703fc7d1d51d61c442de859e0921c0bdd7a
diff --git a/tests/clock-fixture.cpp b/tests/clock-fixture.cpp
index 0b4e04a..d47f695 100644
--- a/tests/clock-fixture.cpp
+++ b/tests/clock-fixture.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil -*- */
 /*
- * Copyright (c) 2013-2020  Regents of the University of California
+ * Copyright (c) 2013-2022  Regents of the University of California
  *                          The University of Memphis
  *
  * This file is part of PSync.
@@ -19,8 +19,7 @@
 
 #include "tests/clock-fixture.hpp"
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 ClockFixture::ClockFixture()
   : m_steadyClock(make_shared<time::UnitTestSteadyClock>())
@@ -50,5 +49,4 @@
   }
 }
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests