Implement a commit helper function for the freetext integration tests

The ink editor integration tests already use a helper function for
committing the editor, so this commit mirrors the approach to the
freetext editor integration tests. This has multiple advantages:

- it improves consistency between the various editor integration tests;
- it makes the code easier to read/understand;
- it reduces code duplication (220 lines of code removed);
- it reduces the number of `getEditorSelector` calls (32 calls removed)
  that contained hardcoded IDs, which helps to isolate the tests and to
  simplify follow-up patches.