test_qthelpplugin: Do not rely on other target creating code from ui file

Summary:
test_qthelpplugin target uses sources which rely on files created from
ui files, but which are only build as part of the target kdevqthelp.

On parallel builds like on KDE CI it can happen that test_qthelpplugin
is build before kdevqthelp, as there is no dependency between the two.
See e.g. https://build.kde.org/view/Kdevelop/job/kdevelop%205.0%20stable-kf5-qt5/PLATFORM=Linux,compiler=gcc/146/console

Adding the respective ui files explicitely to the test_qthelpplugin target
solves the issue.

Reviewers: brauch, apol, kfunk

Reviewed By: kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2906