[TASK] Stabilize ElementsGroupCest by waiting for modal-iframe load

This makes ElementsGroupCest wait, until the modal-iframe is loaded and
the new "block formatting context" (BFC)  is created, before filling any
data into the search field. About BFC also see [1] and [2].

Before this change the search field was sporadically empty on send,
even though no error for `$I->fillField(…)` was triggered.

Additionally this makes some CSS-selectors more specific.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/overflow#description
[2] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_display/Block_formatting_context

Resolves: #105238
Releases: main, 12.4
Change-Id: I311b45f9af700a1aec5b2dfbeca4b1d76c66cc10
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86523


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>