[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/55510


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Reiner Teubner <rteubner@me.com>
Tested-by: Reiner Teubner <rteubner@me.com>
Reviewed-by: Jan Helke <typo3@helke.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>