Add automated test for "Search For Attachments"

The manual test is here: Search For Attachments

Existing automated coverage

None found.
Searched: SearchSuggestIT, SolrSearchIT, SolrIndexerIT, QuickSearchElement/SolrSearchResult page objects, Solr UI page tests and every IT for 'Attachment names'/attachment search; DocumentSolrMetadataExtractorTest only checks attachment indexing.

Missing automated coverage

• Create a terminal and a non-terminal (WebHome) page and attach a uniquely named file to each; wait for the Solr queue
• Search the attachment names in the quick search and assert both are listed, highlighted, in the 'Attachment names' section
• Search them on the Solr search page and assert they appear as attachment results

Suggested location: xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-test/xwiki-platform-search-test-docker/src/test/it/org/xwiki/search/test/ui/SearchSuggestIT.java

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