[BUGFIX] Properly reset package manager in unit tests

Package manager is a static property in ExtensionManagementUtility
that needs proper reset if a test fiddled with it.
Use the proxy class at places where this is not done yet.

Change-Id: I86d3813e1a4cbcb24c05507923694e498ffcb03d
Resolves: #76402
Releases: master
Reviewed-on: https://review.typo3.org/48421


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>