[TASK] Enable postgres test after fixing setUpFrontendRootPage

A functional test has been disabled for postgres execution due
duplicate key issues using `setupFrontendRootPage` in functional
testcases.

As setUpFrontendRootPage has been fixed in the testing framework,
this test is enabled again.

Commands to test:

$ Build/Scripts/runTests.sh -s functional -p 7.4 -d postgres -k 9.6 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 7.4 -d postgres -k 10 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 7.4 -d postgres -k 11 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 7.4 -d postgres -k 12 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 7.4 -d postgres -k 13 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 8.0 -d postgres -k 9.6 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 8.0 -d postgres -k 10 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 8.0 -d postgres -k 11 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 8.0 -d postgres -k 12 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

$ Build/Scripts/runTests.sh -s functional -p 8.0 -d postgres -k 13 \
typo3/sysext/frontend/Tests/Functional/SiteHandling/MountPointTest.php

Resolves: #95114
Releases: master
Change-Id: I85d74f05e7ef347cd217843c0384d460194fac24
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70897


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