Testthat V3

[VisruthSK]

Swap to latest version of testthat. I haven't looked at the tests themselves, so I don't know if snapshots are appropriate anywhere--but 3e would allow for parallel testing which should be a big help. Also, bumping to the latest edition is probably a good idea as a matter of practice.

[jgabry]

We currently use visual snapshots via the vdiffr package (it creates new SVGs of the plots and compares them to existing SVGs). Otherwise I'm not sure we need snapshots. But parallel testing would be nice!

[VisruthSK]

Yeah I didn't really think there'd be too many text based tests. Because of that, I'm hoping that swapping to 3e shouldn't be too much work and should have high ROI wrt test times.