[TASK] Stabilize FileClipboardCest (again)

This one was still flaky. Properly waiting for
the element should finally fix this, this patch
now had a huge number of successful runs locally,
while it failed repeatedly without.

Resolves: #101487
Related: #101440
Releases: main, 12.4, 11.5
Change-Id: I2cf4b3e878caf430ead27340694a169f98e710e8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80243


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>