[TASK] Extend functional tests with linked paths

Extends the functional tests with the possibility to link folders or
files inside the TYPO3 CMS test instance created within typo3temp.
For example you are able to link a folder inside the fixture data of
the functional tests to the fileadmin folder of the test instance.

Resolves: #55111
Releases: 6.2
Change-Id: Ic41809e7642756b7c5eb1e0d717ef3cbd6d81e1d
Reviewed-on: https://review.typo3.org/26904
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader