Improve Export Test Coverage

[ColinBuyck]

Currently, we only have tests to validate that the csv/zip is downloaded for the user/listings export, but we need to also implement tests that verify the content of these downloads.

Technical Notes:

• Part of our backend tests
• Include test coverage of the following: exportFromObject() function for the user export, and exportListingsFromObject() and exportUnitsFromObject() for the listings object

[ludtkemorgan]

This ticket should be moved to bloom core now that it is also in core