[BUGFIX] Fix incomplete mocking in several tests

IndexerServiceTest and ExtensionManagementServiceTest
both execute emit signal methods which in turn
try to instantiate the SignalSlotDispatcher,
which can fail.

Resolve this by mocking these methods.

Resolves: #57206
Releases: 6.2
Change-Id: I0a239abeb7c94839a413870c58f22ae59fed8470
Reviewed-on: https://review.typo3.org/28666
Reviewed-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn