selftests/nolibc: allow report with existing test log

After the tests finish, it is valuable to report and summarize with
existing test log.

This avoid rerun or run the tests again when not necessary.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>