[TASK] Unit tests to addPlugin()

Add unit tests to ExtensionManagementUtility::addPlugin() verifying
the new third parameter $extensionKey is handled properly.

Change-Id: I0257755d0708f12141c78fb97e6e342daf7a6281
Resolves: #60110
Related: #60109
Related: #59414
Related: #59770
Related: #59468
Releases: 6.2, 6.3
Reviewed-on: https://review.typo3.org/31335
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters