[BUGFIX] Reactivate preset actions in current EXT:impexp

This is a regression introduced with #94772 that causes all actions
related to export presets to fail because the controller checks $_GET
instead of $_POST for incoming preset action parameters.

Reduced redundancy in EXT:impexp acceptance tests by introducing
AbstractCest::selectInContextMenu() for basic context menu handling
and moving waitForAjaxRequestToFinish() to the central AbstractCest.
In addition, the declarations of local variables have been bundled.

Resolves: #94938
Releases: master
Change-Id: I13f506d9c32b09b13034388908d710c284885f56
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70684


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>