Add test for additional form actions

Summary: This tests the newly added additional widget actions.

Test Plan:
Passes for me. Is my first GUI test, maybe the timing
needs to be slower for slower machines?

Reviewers: aacid

Reviewed By: aacid

Subscribers: okular-devel

Tags: #okular

Maniphest Tasks: T8627

Differential Revision: https://phabricator.kde.org/D13170