[TASK] Restructure nighly test plan spec

There will be more tests with other PHP versions and DBMS versions
in the future, a little cleanup round will ease the integration.

Resolves: #89783
Releases: master, 9.5, 8.7
Change-Id: I84eb9a813087d8f94cf2800b30efd7dd0174171d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62435


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>