[TASK] Raise typo3/testing-framework:^6.15.0

Brings a couple of backports from main branch that
help with keeping tests compatible with core v12
a bit longer to avoid backport conflicts in this
area for a while.

composer req --dev typo3/testing-framework:^6.15.0

Change-Id: Idb991896d05f478bfcde4e99bd1037e5040c72b5
Resolves: #96109
Releases: 11.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72331


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>