[BUGFIX] Fix incomplete mocking in ExtensionManagementUtilityTest

Some tests rely on actual extensions TCA being present.
Mock the method that reads the TCA files.

Resolves: #57700
Related: #57487
Releases: 6.2
Change-Id: I66cce578983fafc1bfe5a29cc0387b2e3d749873
Reviewed-on: https://review.typo3.org/29221
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel