[BUGFIX] Check extensions in Install Tool broken

Since merging of 64c46b918ef016bdd1d799c5ca06e868d601f692, TYPO3_LOADED_EXT
is no longer an ArrayAccess but a regular array (like pre 6.2). This leaded
to an error in the Extension Manager important action "Check Extension",
causing an endless loop.

Catchable fatal error: Argument 2 passed to
TYPO3\CMS\Install\Controller\Action\Ajax\ExtensionCompatibilityTester
::loadExtLocalconfForExtension() must implement interface ArrayAccess,
array given

Resolves: #56952
Releases: 6.2
Change-Id: I02e83a8221653d62010952f84c17a8f6f6e185ea
Reviewed-on: https://review.typo3.org/28437
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny