selftests/nolibc: use a pipe to in vfprintf tests

Not all architectures implement lseek(), for example riscv32 only
implements llseek() which is not equivalent to normal lseek().
Remove the need for lseek() by using a pipe instead.

Link: https://lore.kernel.org/r/20241221-nolibc-rv32-v1-3-d9ef6dab7c63@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>