Add snapshot tests for output

[ddsjoberg]

https://youtu.be/r3yklH7Gk1s?t=3070

This will be helpful for latex output. I am skeptical whether it'll be good for gt since the HTML seems to get small updates with each release.

Could be used with rmarkdown::render() to ensure the knitted files are as expected.

Thanks @shannonpileggi !

[shannonpileggi]

More info on the {testthat} 3.1.0 release here: https://www.tidyverse.org/blog/2021/10/testthat-3-1/