[nexuist]

If you think about it, every video listed as a search result is a title/thumbnail test, except instead of an A/B test it's A/B/C/D/E...

[TchoBeer]

I think GP meant A/B for your own thumbnails, so you'd be able to randomly show two different thumbnail/titles to viewers and then see stats for each one. They can come clarify if that's incorrect.