test: fs: enable fstab devicetree integration for fatfs

Added tests in order to verify the functionality of the
newly added fatfs fstab integration.

Signed-off-by: Carlo Kirchmeier <carlo.kirchmeier@zuehlke.com>
Co-authored-by: Carlo Kirchmeier <carlo.kirchmeier@zuehlke.com>
Co-authored-by: Dominik Ermel <dominik.ermel@nordicsemi.no>