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