Fix load & scroll event usage in ScrollSpy tests (#19986)

Fix load & scroll event usage in ScrollSpy tests (#19986)

* Use $.one() instead of $.on() since there are no $.off()s in the code.
* Remove unnecessary namespacing of listeners for the `scroll` & `load` events.
    These are vanilla DOM events (not custom jQuery namespaced events)
    and we're not using jQuery namespacing to manage these event listeners either (e.g. `$.off()`).

[skip validator]