kselftest: signal all child processes

We have some many cases that will create child process as well, such as
pidfd_wait. Previously, we will signal/kill the parent process when it
is time out, but this signal will not be sent to its child process. In
such case, if child process doesn't terminate itself, ksefltest framework
will hang forever.

Here we group all its child processes so that kill() can signal all of
them in timeout.

Fixed change log: Shuah Khan <skhan@linuxfoundation.org>

Suggested-by: yang xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>