update testthat version

[donyunardi]

We need to bump up testthat version because we're using testthat::it function in unit tests.

This function was introduced in 3.1.8:
https://github.com/r-lib/testthat/blob/v3.1.8/R/describe.R#L89

This update will fix the Dependency Test - min isolated:
https://github.com/insightsengineering/teal.data/actions/runs/12532231366/job/34950746107#step:9:774

[donyunardi]

Got another failure about compare_proxy which seems to be related to waldo package:
https://github.com/insightsengineering/teal.data/actions/runs/12605104075/job/35133232553#step:9:235

Updating testthat to 3.2.0

[donyunardi]

Still fails:
https://github.com/insightsengineering/teal.data/actions/runs/12605422042/job/35134030219#step:9:853

This may have something to do with the default_cdisc_join_keys object.

[donyunardi]

Issue still persist. I think it may have something to do with other dependencies based on the min_isolated strategy.

@averissimo @m7pr
Any chance you can shed a light on this based on your experience working with verdepcheck?
For sure we need to update the testthat version to 3.1.8 but not sure how to address this compare_proxy issue.