Upgrade `testthat` to 3rd edition

From https://github.com/insightsengineering/coredev-tasks/issues/154

To-Do:

- Add >= 3.0.0 to testthat version in DESC file
- Add to DESC file:
```
Config/testthat/edition: 3
Config/testthat/parallel: true
```
- Fix all errors and warnings in tests -> for example using the above tern has 37 failures and 1118 passes (with 150 warnings)
- Update testthat/testthat.R at the moment locally it runs ParallelReporter (using default_reporter() which is also parallel seems nicer to me) and on CI it uses JunitReporter which is not parallelizable - do we want it parallel there?

Repos:

- [ ] tern https://github.com/insightsengineering/tern/issues/656 (parallel remains)
- [ ] teal
- [ ] ...