upkeep: Remove snapshot test

[jacobvjk]

snapshot tests are a hindrance to submitting updates of the r2dii.* suite to CRAN, because of interdependencies between the r2dii packages that may affect the snap results.

we want to remove them.

AB#11115

[jdhoffa]

Seems like a duplicate of #472 ? (or at least very related)

EDIT: a subset of #472

and thus also AB#9906

[jacobvjk]

this would be good to implement, but is not a blocking priority for the next release.