[BUGFIX] Localization ignores extensionName argument for TypoScript labels

Localization utility should pass extesnionName to configurationManager
when fetching TypoScript _LOCAL_LANG labels.
Without this parameter configurationManager returns configuration
of the current plugin.
For example having
<f:translate key="foo" extensionName="news" />
in the blog_example extension template should load translation
from "news" extesnion.

This patch also makes LocalizationUtility::loadTypoScriptLabels()
testable.

Resolves: #46042
Releases: 4.7, 6.0, 6.1
Change-Id: If3785617defe2894cf3a8dc201e66acd182c18c0
Reviewed-on: https://review.typo3.org/19908
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn