t/helper: merge test-drop-caches 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

@@ -12,6 +12,7 @@ static struct test_cmd cmds[] = {
 	{ "ctype", cmd__ctype },
 	{ "date", cmd__date },
 	{ "delta", cmd__delta },
+	{ "drop-caches", cmd__drop_caches },
 	{ "lazy-init-name-hash", cmd__lazy_init_name_hash },
 	{ "sha1", cmd__sha1 },
 };