tests: add kill_child test

tests: add kill_child test

This tests repeatedly creates and kills children, so some corner
cases in handling of not-quite-existing processes can be observed.

Previously, strace was crashing in the following situation:

    13994 ????( <unfinished ...>
    ...
    13994 <... ???? resumed>) = ?

as tcp->s_ent wasn't initialised on syscall entering and
strace.c:print_event_exit segfaulted when tried to access
tcp->s_ent->sys_name.

* tests/kill_child.c: New file.
* tests/kill_child.test: New test.
* tests/.gitignore: Add kill_child.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(MISC_TESTS): Add kill_child.test.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>