[TASK] Duplicate method in base tests

extbase BaseTestCase extends core BaseTestCase and overrides
inject() with a duplicate. Remove dupe.

Change-Id: I96ce01c9754ce57cb5d10e9a322a8c5acb950647
Resolves: #55606
Releases: 6.2
Reviewed-on: https://review.typo3.org/27272
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn