Fixed #23929 -- Added more tests for create_default_site.

Please see [#23929](https://code.djangoproject.com/ticket/23929#ticket).
We have more places in the test suite that could benefit from something like this, so let's open a ticket and discuss the general issue. I'd like to avoid introducing a new API (not sure if it makes sense to make it public or not) in a patch like this.