Fix extensions panel style (#29273)

* Stop empty state scrolling

* Fix separator inset, separator vertical spacing and margin above the add extensions button

* Add screenshot for add extensions button spacing

* Add variable for add extensions overlap

* use data-testid

* Fix snapshots

src/components/structures/RightPanel.tsx

@@ -282,7 +282,7 @@ export default class RightPanel extends React.Component<Props, IState> {
         }
 
         return (
-            <aside className="mx_RightPanel" id="mx_RightPanel">
+            <aside className="mx_RightPanel" id="mx_RightPanel" data-testid="right-panel">
                 {card}
             </aside>
         );