file-list database is not tested anymore

There has been a test case in bob/bio/base/test/dummy/filelist.py that has been removed during the restructuring. This test case was not only testing the interface to the filelist database (which is now bob.db.bio_filelist), which is non-trivial, but it also served as an example, how to create a filelist database.

With the new database interface, do you think that there is an easy way to use your own (e.g. file list based) database with the bob.bio interface? Shall we re-add a generic BioFileList interface that would make it easy to simply add a user-database based on bob.db.bio_filelist?

At the end, there are still files left (inside https://gitlab.idiap.ch/bob/bob.bio.base/tree/master/bob/bio/base/test/data/atnt) that were used only in the old file-list database interface, which should either be revived, or removed.