[TASK] Add more acceptance tests for extension manager

Refactor the extension manager acceptance tests and add some more.

Change-Id: I72eed4cba52bf6d2451462d9a9b919cff6de20e9
Resolves: #75661
Releases: master
Reviewed-on: https://review.typo3.org/47702


Reviewed-by: Dirk Jüttner <dirk.juettner@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Dirk Jüttner <dirk.juettner@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>