Introduce a helper function to create a freetext editor in the integration tests

Doing so has a number of advantages:

- it removes code duplication, thereby improving readability;
- it removes hardcoded editor IDs, by using the `getNextEditorId` helper
  function that was previously introduced for the highlight editor
  integration tests, thereby improving readability and reusability;
- it removes potential for intermittent failures by not proceeding until
  the freetext editor is fully created and all assertions pass, which
  didn't happen consistently before because the code wasn't centralized.

Author: timvandermeij