ping: set CanBePrefix=false

This commit also sets CanBePrefix in ndndump and ndnpoke tests.

refs #4581

Change-Id: Ie91839426acc02f1879495312ad7ef131e8dac57
diff --git a/tests/ping/server/ping-server.t.cpp b/tests/ping/server/ping-server.t.cpp
index 84e967f..1fe5b35 100644
--- a/tests/ping/server/ping-server.t.cpp
+++ b/tests/ping/server/ping-server.t.cpp
@@ -52,6 +52,7 @@
         .append(to_string(seq));
 
     return Interest(name)
+           .setCanBePrefix(false)
            .setMustBeFresh(true)
            .setInterestLifetime(2_s);
   }