t/helper: merge test-write-cache 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-tool.c

@@ -41,6 +41,7 @@ static struct test_cmd cmds[] = {
 	{ "subprocess", cmd__subprocess },
 	{ "urlmatch-normalization", cmd__urlmatch_normalization },
 	{ "wildmatch", cmd__wildmatch },
+	{ "write-cache", cmd__write_cache },
 };
 
 int cmd_main(int argc, const char **argv)