[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
