Remove mx_HeaderButtons class and add a test (#10713)

* Remove `_HeaderButtons.pcss`

Now that mx_HeaderButtons class is no longer necessary as ARIA "tabpanel" role has been removed from it, it is possible to remove both the class and the style rule for it.

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Tests: add `room-header.spec.ts` and edit the existing tests

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Edit snapshot widths settings

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Fix typo

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Edit a comment

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Optimize a test to check default buttons

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Check highlighted buttons

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Loop

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

---------

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

src/components/views/right_panel/HeaderButtons.tsx

@@ -98,6 +98,6 @@ export default abstract class HeaderButtons<P = {}> extends React.Component<IPro
     public abstract renderButtons(): JSX.Element;
 
     public render(): React.ReactNode {
-        return <div className="mx_HeaderButtons">{this.renderButtons()}</div>;
+        return this.renderButtons();
     }
 }