Add automated test for "Export PDF using User Browser"

The manual test is here: Export PDF using User Browser

Existing automated coverage

• PDFExportIT#configurePDFExport: Administer Wiki > Content > PDF Export: picks a value in the Generator select and saves, but only "Chrome Docker Container".
• PDFExportIT#exportAsPDF: More Actions > Export > PDF, page ticked in the tree, Export, then Export in the options modal; PDF content verified (server-side Chrome generator).
• PDFExportIT#officeMacro: PDF export of a page displaying office content (server-side generator).
• PDFExportJobTest#runClientSide: Unit test of the PDF export job when the PDF is generated client-side (in the user browser).

Missing automated coverage

• Log in as Admin, select "User Browser" in Administer Wiki > Content > PDF Export > Generator and save
• Go to a page created with the Office Importer, open More Actions > Export > PDF with the page ticked, click Export then Export
• Assert the client-side print preview loads and shows the office-imported content with the paged layout
• Assert the browser print is triggered (e.g. stub window.print and check it was called), since the native print dialog cannot be driven

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).