Upgrade to testthat 3e

[MichaelChirico]

I was aiming to send along a PR dropping the {mockery} dependency but it's a bit of a bigger project than at first glance despite the small footprint of {mockery}:

cpp11/tests/testthat/test-utils.R

Line 28 in dc93fb8

mockery::stub(stop_unless_installed, "requireNamespace", FALSE)

cpp11/tests/testthat/test-vendor.R

Line 4 in dc93fb8

mockery::stub(cpp_vendor, "system.file", "")

Because currently {cpp11} uses a lot of describe(..., it(...)) style tests that require more lift to move to 3e.

Filing this issue as an earmark that it would be something nice to have.

[pachadotdev]

I implemented multiple changes that include this request here: #405

[pachadotdev]

hi @DavisVaughan
I wanted to follow up on this. I hope this PR is useful. #405