[R] upgrade to testthat 3e #28602
Description
With the 3rd edition we get a few niceties like expect_snapshot().
We already did some (the majority?) of the work to be compatible with it in ARROW-11752
Reporter: Jonathan Keane / @jonkeane
Assignee: Neal Richardson / @nealrichardson
PRs and other links:
Note: This issue was originally created as ARROW-12871. Please see the migration documentation for further details.