[TASK] Use EXTBASEPLUGIN in tests

Use the new v12 EXTBASEPLUGIN in TS setup
to streamline frontend functional tests a bit.

Resolves: #101773
Related: #100293
Releases: main, 12.4
Change-Id: I84e3cc93f617e3d8feeb76e6acf2c02bc4070a6d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80683


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