fdo#70807: UT to verify used styles in document

Change-Id: Ief2eef638bc3f9e44358addf0d1c655a790834a2
Reviewed-on: https://gerrit.libreoffice.org/7351
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>