[BUGFIX] Fix wrong path handling in DocumentationFile

Currently several unit tests are failing on Windows system due to the
usage of DIRECTORY_SEPARATOR. This patch ensures unix-like paths are
used everywhere and tests are running again.

Resolves: #79759
Releases: master
Change-Id: I60d8e2c31b56fae627c59b8aa4e8fed0eb85b0a9
Reviewed-on: https://review.typo3.org/51637


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Christian Matthes
Tested-by: Christian Matthes
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>