[TASK] Use "EXT:" syntax in setUpFrontendRootPage() tests

Last usage of <INCLUDE_TYPOSCRIPT is the testing
framework. To allow switching to @import, we switch
to remaining places to "EXT:" syntax which is
allowed in @import as well.

Resolves: #101952
Releases: main
Change-Id: I434b55db1fc95b7648008a2dacea36d7e565ce86
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81085


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