tests: Fix compilation errors because of changes ndn-cxx library

Change-Id: I10fb7eb366a03c2e06e6d2efefdde8a9dd0da724
Refs: #2951
diff --git a/tests/core/simple-notification.hpp b/tests/core/simple-notification.hpp
index 64057e3..aad86e6 100644
--- a/tests/core/simple-notification.hpp
+++ b/tests/core/simple-notification.hpp
@@ -53,10 +53,7 @@
   wireEncode() const
   {
     ndn::EncodingBuffer buffer;
-    ndn::prependByteArrayBlock(buffer,
-                               0x8888,
-                               reinterpret_cast<const uint8_t*>(m_message.c_str()),
-                               m_message.size());
+    prependStringBlock(buffer, 0x8888, m_message);
     return buffer.block();
   }