XWIKI-9527: Sheets are not applied on preview action if the document is new
* Enhance the test page objects to be able to write a functional test.
* Make InlinePage#getForm() public to be able to create an ObjectEditPane from it.
* Add PreviewEditPage#getContent() to be able to assert the content in preview mode.
(cherry picked from commit c440c3164dc5387631247504f9f1c6e40b1c64b3)