[BUGFIX] Ensure proper functional test setup for FileBackendTest

With the patch resolving #98931 and #98734 the FileBackendTest
has been migrated from unit to a functional test, and thus now
operating on real filesystem. Used directory listing does not
ensure proper sorting on all systems and used PHP versions,
thus the used expectation may be vary depending on the system.

This change tightens the test for all systems by ensuring a
deterministic test setup, using correct file identifiers for
mocked value returns.

Adding missing parent tearDown() call is added as a side-change,
to ensure full test stack and avoid eventually side-effects in
the future.

Resolves: #98955
Related: #98931
Related: #98734
Releases: main
Change-Id: I668973ae45cf7a4ed07c73e72fd2b2a68dc881ca
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76325


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>