Add automated test for "Add and delete an attachment"

The manual test is here: Add and delete an attachment

Existing automated coverage

• AttachmentIT#uploadAttachments: on a nested (WebHome) test page, uploads attachments from the Attachments tab, deletes one with its Delete button and the confirmation modal, and asserts it is gone, also after reload

Missing automated coverage

• On a nested page (WebHome) and on a terminal page, upload an attachment from the Attachments tab
• Open the More Actions menu and click "Attachments" (id tmAttachments, needs a page-object method); assert the attachments viewer (viewer=attachments) is displayed
• Click the Delete button of the attachment, confirm with "Delete" and assert the attachment is removed

Suggested location: xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-test/xwiki-platform-flamingo-skin-test-docker/src/test/it/org/xwiki/flamingo/test/docker/AttachmentIT.java

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