Use more of testthat 3e functionality

#289 bumps testthat to the third edition for snapshots. There are potentially other parts of the test suite which could be updated to use new functionality introduced in 3e. Moving away from `context()` and some of the now deprecated functions would probably be the bulk of this issue.

https://testthat.r-lib.org/articles/parallel.html

https://testthat.r-lib.org/articles/third-edition.html#changes