Replace magic constants with macros.

Makes strlen_s limits clearer.

Change-Id: Iaa7488bdada389f5e50eea40091c541a360ae933
diff --git a/test/hftest/inc/hftest.h b/test/hftest/inc/hftest.h
index 703176c..145c2ed 100644
--- a/test/hftest/inc/hftest.h
+++ b/test/hftest/inc/hftest.h
@@ -66,6 +66,7 @@
 #define FAIL(...) HFTEST_FAIL(true, __VA_ARGS__)
 
 /* Service utilities. */
+#define SERVICE_NAME_MAX_LENGTH 64
 #define SERVICE_SELECT(vm_id, service, send_buffer) \
 	HFTEST_SERVICE_SELECT(vm_id, service, send_buffer)