fix: improve test_use_for_loop to check if the iterator is used

[Wolfy7]

Please check if the PR fulfills these requirements:

• [x] The commit message follows our guidelines.
• For bug fixes and features:
  • [x] You tested the changes.

Related issue (if applicable): #920

What kind of change does this PR introduce?

Improvement of test_use_for_loop (course\lesson-21-strings\string_array\TestsStringArray.gd) to check that the iterator is used.

New feature or change

What is the current behavior?

See #920

What is the new behavior?

The test will fail with message that the iterator was not used: