kselftests/sched: cleanup the child processes

Previously, 'make -C sched run_tests' will block forever when it occurs
something wrong where the *selftests framework* is waiting for its child
processes to exit.

[root@iaas-rpma sched]# ./cs_prctl_test

 ## Create a thread/process/process group hiearchy
Not a core sched system
tid=74985, / tgid=74985 / pgid=74985: ffffffffffffffff
Not a core sched system
    tid=74986, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74988, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74989, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74990, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
    tid=74987, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74991, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74992, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74993, / tgid=74987 / pgid=74985: ffffffffffffffff

Not a core sched system
(268) FAILED: get_cs_cookie(0) == 0

 ## Set a cookie on entire process group
-1 = prctl(62, 1, 0, 2, 0)
core_sched create failed -- PGID: Invalid argument
(cs_prctl_test.c:272) -
[root@iaas-rpma sched]# ps
    PID TTY          TIME CMD
   4605 pts/2    00:00:00 bash
  74986 pts/2    00:00:00 cs_prctl_test
  74987 pts/2    00:00:00 cs_prctl_test
  74999 pts/2    00:00:00 ps

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chris Hyser <chris.hyser@oracle.com>
Link: https://lore.kernel.org/r/20210902024333.75983-1-lizhijian@cn.fujitsu.com