selftests/mm: fix assumption that sudo is present

selftests/mm: fix assumption that sudo is present

If we are root, sudo isn't needed.  If we are not root, we need sudo, so
skip the test if it isn't present.

We already do this for on-fault-limit, but this uses separate
infrastructure since that is specifically for sudo-ing to the nobody user.

Note this ptrace_skip configuration still fails if that file doesn't
exist, but in that case the test is still fine, so this just prints an
error but doesn't break anything.  I suspect that's probably deliberate.

Link: https://lkml.kernel.org/r/20250221-mm-selftests-v2-2-28c4d66383c5@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>