Test against pinned testthat version to skirt withr->3.6.0 dependency

[MichaelChirico]

#83 started testing R>=3.6.0 in GHA, because {testthat} requires {withr} requires R>=3.6.0 at their current versions.

The easiest approach for now is to pin {testthat} to a slightly older version & test with that. Here's {testthat} 3.0.0 (our current stated minimum version), so we can IVT our way to getting a nice recent-ish {testthat} that also works on 3.3.0.

Depends: R (>= 3.1)
Imports: brio, callr (>= 3.5.1), cli, crayon (>= 1.3.4), desc, digest,
        ellipsis (>= 0.2.0), evaluate, jsonlite, lifecycle, magrittr,
        methods, pkgload, praise, processx, ps (>= 1.3.4), R6 (>=
        2.2.0), rlang (>= 0.4.1), utils, waldo (>= 0.2.1), withr (>=
        2.0.0)