EZP-21213: Fixed Image FieldType integration tests

This required disabling the file_exists() test that is done on
created data: the base integration test wipes database data
between every test, and filesystem data before and after each
test class.

This is globally fine, except that for images, it happens that
since with some databases (e.g. not MySQL), the autoincrement
counter is reinitialized when the tables are empties. The
consequence is that IDs are reused for Field and Version, and
we end up with the same filename.

This was fine with without this branch, since storeFieldData()
will re-use a file with the same path. But the publish event
handler won't, and shouldn't.

Images should not be re-used because the generated path is
identical, but only when a Field is provided with existing data.