[BUGFIX] Fix PHP 8.0 warning with inline sychronize

Scenario:
* Add a content element with an image in default language.
* Translate the content element to some other language,
  this will create the image in the localized element, too.
* Edit the default language element and add another image.
* Edit the localized element, see the 'shadowed' new image,
  click "localize" button on the right.
-> Red error pop up, logged warning: "Array to string
conversion" in FormInlineAjaxController.php

The patch fixes this, and adds two acceptance tests to
have a basic coverage for the new content element wizard,
the element browser inline image selection, and the
localization 'synchronize' process.

Change-Id: I9c930d3b7b625b96b6321c14c455d0e62f70b5dc
Resolves: #97090
Releases: main, 11.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73803


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>