Update to testthat3

[cderv]

It is probably a good idea to update the tests suites to a more modern usage.

• Some of the tests still use the old form expect_that() and equals().
• if(require instead of skip_if_not_installed()
• context() is still used
• test_path() is not used for files, which makes it less easy to locally tests interactively.

I can do that at some point.