Move most of UnencryptedMessageTest tests to MimeTreeParser.
Summary:
As most of those tests in UnencryptedMessageTest do not need anything from
MessageViewer we should move them to MimeTreeParser in order to find
those easier. Some smaller syntactical updates are done.
Reviewers: KDE PIM
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D20750