Add automated test for "Export Office page as ODT"

The manual test is here: Export Office page as ODT

Existing automated coverage

• OfficeExporterIT#exportODT: Exports a page to ODT through the export URL (format=odt) and checks with Tika that the result is an ODT document; the page only contains the word "content".
• OfficeImporterIT#verifyImport: Precondition: creating pages from office documents (ODT, DOC, etc.) with the Office Importer.

Missing automated coverage

• Import an office document (e.g. one with headings, a table and an image) with the Office Importer to create the page
• On that page open More actions > Export, select ODT in the export modal (ExportModal#exportAs) instead of calling the export URL
• Download the ODT and assert its content (text, headings, table cells, embedded image) matches the wiki page, not only its MIME type

Suggested location: xwiki-platform-core/xwiki-platform-office/xwiki-platform-office-test/xwiki-platform-office-test-docker/src/test/it/org/xwiki/officeimporter/test/ui/OfficeExporterIT.java

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