[libc] fixup ftello test (#87282)

Use a seek offset that fits within the file size.

This was missed in presubmit because the FILE based stdio tests aren't
run in
overlay mode; fullbuild is not tested in presubmit.

WRITE_SIZE == 11, so using a value of 42 for offseto would cause the
expression
`WRITE_SIZE - offseto` to evaluate to -31 as an unsigned 64b integer
(18446744073709551585ULL).

Fixes #86928