graph_editor: fix a flaky test by adding a delay

as explained in comment, this API is asynchronous but the promise to await is
not exposed

bt5/erp5_graph_editor/SkinTemplateItem/portal_skins/erp5_graph_editor/dream_graph_editor/jsplumb/test.js.js

@@ -296,7 +296,7 @@
                 jsplumb_gadget.render(sample_data_graph);
             }).then(runTest).fail(error_handler).always(start);
         });
-        test("Node can be connected", function () {
+        test("Nodes can be connected", function () {
             var jsplumb_gadget;
             stop();
             function runTest() {
@@ -308,6 +308,9 @@
                         source: node1.id,
                         target: node2.id
                     });
+                    // .connect insternal API is asynchronous, but there's no way to wait for the event to be processed.
+                    // for now we just wait for a short delay
+                    return RSVP.delay(1e3);
                 }).then(function () {
                     return jsplumb_gadget.getContent();
                 }).then(function (content) {