Exercise repeated close during the server link teardown event

tests/test_etcp_stcp.c

@@ -306,6 +306,10 @@ static void close_from_link_event(struct ETCP_CONN *conn, struct ETCP_LINK *link
         old_state, old_status, link->link_state, link->link_status);
     if (test->action == 2) etcp_connection_close(conn);
     else if (test->action == 3) stcp_link_close(link->tcp_link);
+    else if (test->action == 4 && link->closing) {
+        test->nested_close++;
+        etcp_link_close(link); etcp_link_close(link);
+    }
     else etcp_link_close(link);
 }