waitid/waitid02.c: Add new ECHILD errno test

* Add new ECHILD errno test for waitid(2) idtype and id (waitid()) is
  not a child of the calling process.

  NOTE: The test _does_ fork so that the process calling waitid() has at
        least one unwaited for child (otherwise the it may return ECHILD
        just because there is not child to be waited for).

* Cleanup: remove an useless tst_exit()

* Add tst_count = 0

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Acked-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>