[BUGFIX] Stabilize flaky acceptance test

The initial view of the installer enables the 'proceed'
buttons on first step via JS. The ac test has to wait
until the buttons are rendered before proceeding.

Change-Id: I1426f1434d80124ac5036c1270668867c86fc2d6
Resolves: #82516
Related: #82504
Releases: master
Reviewed-on: https://review.typo3.org/54180


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>