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