[BUGFIX] Avoid broken assertion in ExtensionRepositoryTest

$this->count() in phpunit always returns 1 and has no arguments
as correctly found by phpstan. Fix a test to actually assert
subject result count.

Resolves: #103924
Releases: main, 12.4
Change-Id: I396b695eaeaa986a4cd9883e028ed6032d2f6339
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84412


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