The unit tests are built to expect the values from the static API responses in Xero's OpenAPI spec.
https://app.swaggerhub.com/apis/Xero/accounting/2.0.0
If you wanted to modify the tests for your Demo Company you can, you'd start by changing base URL to https://api.xero.com/api.xro/2.0/
new ApiClient("https://api.xero.com/api.xro/2.0/",null,null,null);
But your tests will fail as the data in your Demo Company will not match the static responses. That is one of the challenges of creating tests against a live API. I also found challenges testing values that are unique in Xero. For example a Contact name must be unique, so once I create a contact with the name "John Smith" and test that "John Smith" was returned, the second time you run that test, the API will return a validation error that a contact named "John Smith" already exists.
Not saying, our choice to use swaggerhub virtual server to host our static responses is the only way to test the models, but it's the choice we've made.
Hopefully, this information helps.