rename identity challenge dir to challenge

Change-Id: Ieb89704404dae2086bd83987624670432bb69663
diff --git a/tests/unit-tests/challenge-email.t.cpp b/tests/unit-tests/challenge-email.t.cpp
index 426ea03..34f2089 100644
--- a/tests/unit-tests/challenge-email.t.cpp
+++ b/tests/unit-tests/challenge-email.t.cpp
@@ -18,7 +18,7 @@
  * See AUTHORS.md for complete list of ndncert authors and contributors.
  */
 
-#include "identity-challenge/challenge-email.hpp"
+#include "challenge/challenge-email.hpp"
 #include "test-common.hpp"
 
 namespace ndn {