ptrace tests: Add a test using PROC_REAP_KILL to kill a traced debuggee

This exercises the bug fix in commit 9241ebc796c1
("thread_single(9): decline external requests for traced or debugger-stopped procs").

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44564

(cherry picked from commit 43b4da44118e4fe29e9d7456db4390c9cbb53636)