selftests/nolibc: try to read from stdin in readv_zero test

selftests/nolibc: try to read from stdin in readv_zero test

When stdout is redirected to a file this test fails.
This happens when running through the kselftest runner since
commit d9e6269e3303 ("selftests/run_kselftest.sh: exit with
error if tests fail").

For consistency with other tests that read from a file descriptor,
switch to stdin over stdout. The tests are still brittle against
a redirected stdin, but at least they are now consistently so.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260106-nolibc-selftests-v1-1-f82101c2c505@weissschuh.net