[BUGFIX] Prepare changed json_response middleware

The json_response functional test extension will soon
avoid returning json encoded responses. This will make
the functional frontend test suite fully transparent
regarding responses.
This patch prepares a couple of frontend functional
tests towards this change.

Resolves: #96112
Releases: master, 11.5
Change-Id: I8116caf08e4b8b76ad5d23b49954d9cb78d1c4de
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72319


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>