Fix min_isolated pipeline

[donyunardi]

We got a failed pipeline in the min_isolated scheduled pipeline:
https://github.com/insightsengineering/teal.code/actions/runs/11537366901/job/32114694527

  ══ Failed tests ════════════════════════════════════════════════════════════════
  ── Error ('test-qenv_get_code.R:745'): (code run outside of `test_that()`) ─────
  Error: 'it' is not an exported object from 'namespace:testthat'
  Backtrace:
      ▆
   1. └─testthat::describe(...) at test-qenv_get_code.R:745:0
   2.   └─base::eval(substitute(code), describe_environment)
   3.     └─base::eval(substitute(code), describe_environment)

It's due to this line:
https://github.com/insightsengineering/teal.code/blob/main/tests/testthat/test-qenv_get_code.R#L746

We need to bump up the version of testthat in DESCRIPTION file.