Ensure we drop the last pane item

Previously, we weren't updating the toolbar's active item when emptying out a pane. This enhances an integration test to ensure that we don't hold references to any editors or buffers once we close everything.

crates/workspace/src/pane.rs

@@ -580,6 +580,7 @@ impl Pane {
                         let item = pane.items.remove(item_ix);
                         if pane.items.is_empty() {
                             item.deactivated(cx);
+                            pane.update_toolbar(cx);
                             cx.emit(Event::Remove);
                         }