Fix load & scroll event usage in ScrollSpy tests

Created by: cvrebert

• Use $.one() instead of $.on() since there are no $.off()s in the code, nor do we expect multiple occurrences of the events.
• 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()).

CC: @XhmikosR @hnrch02 for review