[BUGFIX] Wait until modal overlay is closed in tests

The modal helper should wait until the overlay is closed when
interacting with a modal.

Change-Id: I9efe74d865487ec1d38979f24a764612cb4b8073
Resolves: #75620
Releases: master
Reviewed-on: https://review.typo3.org/47710


Reviewed-by: Dirk Jüttner <dirk.juettner@gmail.com>
Tested-by: Dirk Jüttner <dirk.juettner@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>