selftests/nolibc: prepare /tmp for tests that need to write

create a /tmp directory. If it succeeds, the directory is writable,
which is normally the case when booted from an initramfs anyway.

This will be used instead of procfs for some tests.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Link: https://lore.kernel.org/lkml/20230710050600.9697-1-falcon@tinylab.org/


[wt: removed the unneeded mount() call]
Signed-off-by: Willy Tarreau <w@1wt.eu>