selftests/nolibc: use correct return type for read() and write()

Avoid truncating values before comparing them.

As printf in nolibc doesn't support ssize_t add casts to int for
printing.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>