Invoke the PDF export filter and then use the PDF tokenizer from
xmlsecurity to assert the contents of created PDF file. The testcase
fails with commit 6db0f1fe (tdf#106059
PDF export: create a reference XObject for PDF images, 2017-02-22)
reverted.

Change-Id: I90526fef41d9560ae447f586df766bc50a491c43
Reviewed-on: https://gerrit.libreoffice.org/34609Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>