add minimum sizes to node for restart button test

test/src/notebook/notebook/default-toolbar.spec.ts

@@ -219,6 +219,10 @@ describe('notebook/notebook/default-toolbar', () => {
 
       it('should restart the kernel when the dialog is accepted', (done) => {
         let button = ToolbarItems.createRestartButton(panel);
+        button.node.style.minHeight = '10px';
+        button.node.style.minWidth = '10px';
+        panel.node.style.minHeight = '300px';
+        panel.node.style.minWidth = '300px';
         panel.attach(document.body);
         button.attach(document.body);
         button.node.click();