[TASK] Prevent undefined encryptionKey in tests

With #103046, `encryptionKey` has been removed from
`DefaultConfiguration.php`. This leads to PHP array
key warnings in unit tests.

The patch sets it for various tests.

Resolves: #103148
Related: #103046
Releases: main, 12.4, 11.5
Change-Id: Ife279449c2db569c8d63a0cc19009f1a3ca278d5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83032


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