[TASK] Resolve PHPStan issues in AbstractRecycleTestCase

Mostly these are due to SimpleXML not casting cleanly to strings,
so making the casts explicit resolves them.

Resolves: #97643
Releases: main, 11.5
Change-Id: I036ba8bf08d2a96f5f58b352b82991eeadded990
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74672


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>