zdtm/session00: wait when a child closes fd

A static test should not change its state during C/R

===================== Run zdtm/live/static/session00 in ns =====================
Start test
./session00 --pidfile=session00.pid --outfile=session00.out
Run criu dump
Run criu restore
7: Old files lost: set(['4'])
7: New files appeared: set([])
############# Test zdtm/live/static/session00 FAIL at fds compare ##############
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/live/static/session00.c

@@ -151,6 +151,12 @@ child:
 		pr_perror("read failed");
 		return 1;
 	}
+	/* wait when a child closes fd */
+	ret = read(p[0], &testcases[i].pid, sizeof(pid));
+	if (ret != 0)) {
+		pr_perror("read failed");
+		return 1;
+	}
 
 	close(p[0]);