selftests/bpf: Update bpf_clone_redirect expected return code

Commit 151e887d ("veth: Fixing transmit return status for dropped
packets") started propagating proper NET_XMIT_DROP error to the caller
which means it's now possible to get positive error code when calling
bpf_clone_redirect() in this particular test. Update the test to reflect
that.

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230911194731.286342-2-sdf@google.com