Revert "Removed the file in var/ezdemo_site"

This reverts commit 46621672de27ce63b8ef1fd8993469ed1b4bfe35.
EZP-20321: Changed Handler\Filesystem to use a prefix to get tests to work
EZP-20321: Removed var/files/.keep. We use var/ezdemo_site/ from now on
EZP-20321: Added a workaround to FileBaseIntegrationTest

The same storage folder (var/ezdemo_site) is used by both Repository & FieldType integrationt tests.
FieldType tests create temporary files in the storage directory, and clean them up afterwards.
Repository ones rely on files that match the database fixture (an image at the time of this commit),
and this file must not be removed by the FieldType tests.