Add automated test for "Import Office File"

The manual test is here: Import Office File

Existing automated coverage

• OfficeImporterIT#verifyImport: Office server conversion of doc/ppt/xls/odt/odp files through the Office Importer application UI, with assertions on the imported content and attachments.

Missing automated coverage

• With the office server started, edit a page in CKEditor and open Insert > Import Office File (needs a page object for the Insert menu and the import dialog).
• Upload/select an office document (e.g. msoffice.97-2003/Test.doc) and click Import; assert its text is inserted in the editor.
• Save & View and assert "This is a test document." is in the view content.

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/OfficeImporterIT.java

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