Add automated test for "Export Pdf (Old PDF)"

The manual test is here: Export Pdf (Old PDF)

Existing automated coverage

• PDFExportIT#exportSingleSimplePageAsPDF: Old (FOP) PDF export through bin/export?format=pdf (the export-pdf UI is not installed in this distribution), PDF parsed with PDFBox and text checked.
• PDFExportIT#exportContentWithAttachmentLink: FOP export of a page with an attachment link and an image: link URL and image count asserted.
• PDFExportIT#exportTableOfContents: FOP export with pdftoc=1&attachments=1&pdfcover=0 URL parameters: table of contents links asserted.
• XHTML2FOTest#transformWithTransparentColors: Unit test of the XHTML to FO conversion, input includes the pdfheader block.

Missing automated coverage

• With the new PDF export installed, set Administer Wiki > Content > PDF Export > Replace FOP to No and save
• Create a page with comments and an image attachment
• Open More Actions > Export > PDF (FOP), select Cover, Table of Contents, Header, Footer, Comments and Page Attachments, click Export
• Assert with PDFBox that the cover page, TOC, header and footer text, the comments and the attachment are in the PDF
• Export again with all options deselected and assert only the page content is present (no cover, TOC, header, footer, comments, attachments)

Suggested location: xwiki-platform-core/xwiki-platform-export/xwiki-platform-export-pdf/xwiki-platform-export-pdf-test/xwiki-platform-export-pdf-test-docker/src/test/it/org/xwiki/export/pdf/test/ui/PDFExportIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).