rustdoc: fix flakey test on sidebar resize settings

Author: notriddle

tests/rustdoc-gui/sidebar-resize-setting.goml

@@ -20,5 +20,5 @@ press-key: "Enter"
 wait-for-css: ("#settings", {"display": "block"})
 assert-property: ("#hide-sidebar", {"checked": "true"})
 click: "#hide-sidebar"
+wait-for-css: (".sidebar", {"display": "block"})
 assert-property: ("#hide-sidebar", {"checked": "false"})
-assert-css: (".sidebar", {"display": "block"})