Add test covering update view without queryset attribute

[browniebroke]

Description

Add test covering an expected behaviour from pre 3.15, where update views don't have to define a queryset attribute or get_queryset method. The behaviour regressed in 3.15 and the change was reverted in 3.15.1. However, it's a behaviour users care about which isn't covered by the test suite.

• Original PR: Re-prefetch related objects after updating #8043
• Regression report: 3.15 not backwards compatible with 3.14 - "View' should either include a queryset attribute, or override the get_queryset() method." #9306
• Reverted in Revert "Re-prefetch related objects after updating" #9327
• Test extracted from: Fix UpdateModelMixin to work when no queryset is defined is defined on the view #9314