WIP #21230: Remove some direct settings manipulations in tests

See [ticket 21230](https://code.djangoproject.com/ticket/21230). The ticket is marked as 'easy pickings', however there is quite some work to be done and not everything is [straight-forward](https://code.djangoproject.com/ticket/21230#comment:2). So I started off with some low hanging fruit. This doesn't close the ticket, as there are quite a few remaining places where settings are manipulated.

There might be some problem with code style, especially in places of multiline decorator calls. Please advice on how the patch should be improved.