ui.get_scroll_element: Set up SimpleBar if it’s expected but missing.

Although SimpleBar automatically sets itself up on elements with a
`data-simplebar` attribute, sometimes we try to set event listeners
before that happens.  Create the SimpleBar early in that case.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

frontend_tests/node_tests/buddy_list.js

@@ -16,6 +16,7 @@ function init_simulated_scrolling() {
     });
 
     var elem = {
+        dataset: {},
         scrollTop: 0,
         scrollHeight: 0,
     };

frontend_tests/node_tests/stream_list.js

@@ -340,6 +340,9 @@ run_test('zoom_in_and_zoom_out', () => {
         f.call(elem(stream_li1));
         f.call(elem(stream_li2));
     };
+    $('#stream-filters-container')[0] = {
+        dataset: {},
+    };
     stream_list.set_event_handlers();
 
     stream_list.zoom_in_topics({stream_id: 42});