tests/kernel/t_cloexec: New tests for close-on-exec.

Verified that the following tests fail on a netbsd-10 kernel:

- clonedev_posixspawn
- dup3_posixspawn
- fcntldupfd_posixspawn

PR kern/58822: cl

tests/kernel/t_cloexec: New tests for close-on-exec.

Verified that the following tests fail on a netbsd-10 kernel:

- clonedev_posixspawn
- dup3_posixspawn
- fcntldupfd_posixspawn

PR kern/58822: close-on-exec is broken for dup3 and opening cloning
devices

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