Add tests for the updateOfflineSites function

The `updateOfflineSites` function introduced in PR https://github.com/airqo-platform/AirQo-api/pull/4082 lacks test coverage.

Specific tests should be added for:

* Updating sites as offline when they're not in the active set and beyond the threshold
* Handling empty data arrays
* Verifying the correct MongoDB query conditions are used
* Ensuring the SiteModel update operation is called with correct parameters

The test structure should follow the existing patterns in the file, using the same testing utilities (chai, sinon) and setup/teardown approach.

Refer to the comment: https://github.com/airqo-platform/AirQo-api/pull/4082#discussion_r1885023407

Requester: @Baalmart