[BUGFIX] Stabilize Page Tree acceptance tests

Make sure that the first root node is selected before
the tests are executed.

Resolves: #93471
Releases: master, 10.4
Change-Id: I8d2b973a8d7e0a6f94db39967b8710474502335b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67673


Tested-by: core-ci <typo3@b13.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>