zdtm/mountpoints: wait when a child process opens descriptors

Otherwise we can see this error:
5: Old files lost: set([])
5: New files appeared: set(['5', '6'])

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/mountpoints.c

@@ -27,6 +27,8 @@ struct ns_exec_args {
 	int status_pipe[2];
 };
 
+task_waiter_t t;
+
 int ns_child(void *_arg)
 {
 	struct stat st;
@@ -52,6 +54,8 @@ int ns_child(void *_arg)
 
 	pid = fork();
 
+	task_waiter_complete(&t, 1);
+
 	test_waitsig();
 
 	if (pid) {
@@ -80,6 +84,8 @@ int main(int argc, char **argv)
 
 	test_init(argc, argv);
 
+	task_waiter_init(&t);
+
 again:
 	fs_cnt = 0;
 	f = fopen("/proc/self/mountinfo", "r");
@@ -277,6 +283,8 @@ done:
 		}
 	}
 
+	task_waiter_wait4(&t, 1);
+
 	test_daemon();
 	test_waitsig();