[BUGFIX] Replace calls to the deprecated PHP function each()

This patch replaces the calls to the PHP function each() as it is
marked as deprecated in PHP 7.2. Additionally it adds unit tests for
QueryGenerator::getSubscript().

Resolves: #83737
Releases: master, 8.7, 7.6
Change-Id: Ie61a6d44fcdbd4ce6105a6c185085a3a68866fd8
Reviewed-on: https://review.typo3.org/55570


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>