t/helper: merge test-strcmp-offset into test-tool

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-strcmp-offset.c

@@ -1,6 +1,7 @@
+#include "test-tool.h"
 #include "cache.h"
 
-int cmd_main(int argc, const char **argv)
+int cmd__strcmp_offset(int argc, const char **argv)
 {
 	int result;
 	size_t offset;