feat: improve tampering test on exercise "Authentication System"

[salvatore-piccione]

Tests for Authentication System exercises 4 and 5 can be improved by trying to actually tamper the data to ensure that the expected changes have been done. This PR adds two test cases:

• the former is about the tampering check on the admin
• the latter is about the tampering of developers by replacing an element in the dictionary

Exemplar solution has been updated to be more inline with the exercise request (i.e. getDevelopers() returns an instance of the interface IDictionary despite of an instance of ReadOnlyDictionary).

[github-actions]

Hello. Thanks for opening a PR on Exercism. We are currently in a phase of our journey where we have paused community contributions to allow us to take a breather and redesign our community model. You can learn more in this blog post. As such, all issues and PRs in this repository are being automatically closed.

That doesn't mean we're not interested in your ideas, or that if you're stuck on something we don't want to help. The best place to discuss things is with our community on the Exercism Community Forum. You can use this link to copy this into a new topic there.

---

Note: If this PR has been pre-approved, please link back to this PR on the forum thread and a maintainer or staff member will reopen it.

[salvatore-piccione]

Hello @ErikSchierboom, I saw that checks on Configlet failed. AFAIK this is not due by the changes on this PR since the failure affects other exercises.

[ErikSchierboom]

Thanks!