[TASK] Enable Content-Security-Policy in backend acceptance tests

To avoid introducing new components that rely on inline JavaScript,
Content-Security-Policy HTTP headers for the TYPO3 backend are enabled
when executing automated tests.

Resolves: #95898
Releases: main, 11.5
Change-Id: I2bbc1ce055d066381cd53449d1b37fd2cafe0168
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72137


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>