[TASK] Tests: Decouple InstallTool unit tests from real filesystem

Change the FolderStructure tests to use a vfsStream virtual filesystem
for all tests that work with files, directories and permissions to make
the tests less reliant on the capabilities of the user running the
testsuite and the real filesystem.

Resolves: #71053
Releases: master
Change-Id: Ife42f43008215b382f70d5fe956d3dcf9590f5b9
Reviewed-on: https://review.typo3.org/44301


Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>