[tests] Add Test Cases Ensuring Working of WHOIS storage and retrieval

[DragnEmperor]

Add unit and integration tests to ensure working of WHOIS-related logic. This includes:

• Testing fetch_whois_details task for correct parsing and saving of WHOIS data
• Verifying creation and update behavior of WHOISInfo model
• Ensuring trigger_whois_lookup is called only when last IP is public and has changed
• Adding tests for the periodic refresh task

Dependencies

• [x] https://github.com/openwisp/openwisp-controller/issues/1032
• [x] https://github.com/openwisp/openwisp-controller/issues/1033
• [x] https://github.com/openwisp/openwisp-controller/issues/1037