Improve testPrintD

Summary:

• swap out manual management for a PrintDHelper class that allocates an action and MessageBoxHelper, runs the script, and checks the result.
• use _data() to run the helper with different data; this improves observability as well.

Test Plan: - Run tests, see same results as before

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D23164