[BUGFIX] Loaded Extensions in ExtensionCompatibilityTester

Remove the obsolete call to loadTypo3LoadedExtensionInformation() and
read the loaded Extensions from the TYPO3_LOADED_EXT global.

Additionally the array type hints are replaced with the ArrayAccess
interface since the LoadedExtensionsArray returns instances of
LoadedExtensionArrayElement which implements that interface.

Resolves: #52866
Releases: 6.2
Change-Id: I40db793f2c35b50636c8411aa7a141efc3c90320
Reviewed-on: https://review.typo3.org/24820
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik
Reviewed-by: Markus Klein
Tested-by: Markus Klein