selftest: Rework child process cleanup

selftest: Rework child process cleanup

We now:
 - call gdb_backtrace on the stuck pid to determine why it is stuck
 - cleanup faster as we catch the process exit
   (by not waiting until 1 second after the exit for waitpid() to return -1)

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>