KParts: add unittest for PartLoader, required shuffling things around

Authored by dfaure on Mar 9 2020, 12:48 AM.

Description

KParts: add unittest for PartLoader, required shuffling things around

Summary:
The notepad part was part of the interactive test programs, move it to
autotests. Generate JSON for it. Put XMLGUI file in qrc.
Adapt interactive tests to find the notepadpart the new way.

Test Plan: make && ctest

Reviewers: aacid, nicolasfella, kossebau

Maniphest Tasks: T12173

Differential Revision: https://phabricator.kde.org/D27967