investigate usages of `skip_on_cran()` in tests

[simonpcouch]

This release will correct two breakages from dependencies that we weren't alerted of because we skipped related tests on CRAN. Let's look into why those exist—from what I remember, this was largely due to check time issues. We can probably get those under the limits with some performance optimizations, either in high-traffic machinery here or in modeltests; in either case, that's a win regardless.