[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
mute

Change-Id: I643de2cf0df800617146245cb241d00a41c8bb36