doc: add "doing real test in the child process" paragraph

Add description of the newly added tst_record_childstatus() function
and a paragraph that describes best practices for using tst_* interface
from a child process.

Also fixes all NOTEs that forbids using tst_* interface from child
process.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
1 file changed