test(grantleeheaderstyleplugin): Adopt changes from GrantleeHeaderStyle

Summary:
MessageViewer::GrantleeHeaderStyle was refactored and some things were
changed, those changes need to be adopted to make the tests pass again.

Reviewers: KDE PIM, dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D23865