Add test for read only set and checkbox calculate

Summary:
It's a real world screnario to have checkboxes that toggle
the read only state for other fields.

The test is a parttest because we need to trigger activiation
action when changing the field, this is handled through the
formwidgetscontroller.

Reviewers: Okular

Tags: Okular

Maniphest Tasks: T8097

Differential Revision: https://phabricator.kde.org/D10869