tests/jaildesc: Add a couple more test scenarios

[markj]

@jamie the self_destruct test passes but leaves the jail in the dying state, so I'd guess there's a credential leak somewhere. Could you please take a look?

root@freebsd:/usr/tests/sys/kern # kyua test jaildesc
jaildesc:curjail_get  ->  passed  [0.008s]
jaildesc:poll_close_race  ->  passed  [0.009s]
jaildesc:poll_close_race_get_desc  ->  passed  [0.010s]
jaildesc:poll_remove_wakeup  ->  passed  [0.010s]
jaildesc:self_destruct  ->  passed  [0.010s]

Results file id is usr_tests_sys_kern.20260521-141216-979314
Results saved to /root/.kyua/store/results.usr_tests_sys_kern.20260521-141216-979314.db

5/5 passed (0 broken, 0 failed, 0 skipped)
root@freebsd:/usr/tests/sys/kern # jls -vd
   JID  Hostname                      Path
        Name                          State
        CPUSetID
        IP Address(es)
     5                                /
        jaildesc_self_destruct        DYING
        3

[markj]

Update a helper function name