tests/sys/file: add tests for the lowest fd after fork

[maxim]

Summary

fdcopy() skips plain kqueues and close-on-fork descriptors when it
copies the table into the child. Check that dup(2) in the child still
returns the lowest unused descriptor, both when the skipped descriptor
is the lowest free slot and when it sits above an existing hole.

Test cases:

kqueue_no_hole      a plain kqueue is the lowest descriptor; the
                    child's dup() reuses its slot
kqueue_two          two adjacent plain kqueues; the child's dup()
                    returns the lower one
kqueue_above_hole   a plain kqueue sits above a closed descriptor;
                    the child's dup() returns the hole
kqueue_cponfork     a KQUEUE_CPONFORK kqueue is copied; the child's
                    dup() returns the next slot
clofork_no_hole     an O_CLOFORK descriptor is the lowest one; the
                    child's dup() reuses its slot
clofork_above_hole  an O_CLOFORK descriptor sits above a closed
                    descriptor; the child's dup() returns the hole

Without 99a58803f9a3, kqueue_no_hole, kqueue_two and
clofork_above_hole fail.

PR: 300234
MFC after: 1 week
MFC with: 99a58803f9a3
Co-authored-by: Claude Code Opus 5.5

[maxim]

add Co-authored-by

[kib]

Yes, the case of 'kqueue_above_hole a plain kqueue sits above a closed descriptor; the child's dup() returns the hole' is the most illustrative.
Basically, the issue was the following: we have fds 0, 1, 2, 3<closed>,4<nocopy kqueue>, Then fork leaves fd_freefile at 4.