selftests/nolibc: prepare: create /dev/zero

/dev/zero is commonly used to allocate anonymous memory, it is a very
good file for tests, let's prepare it.

Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20230702193306.GK16233@1wt.eu/


Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>