Missing test for `checkfiles`

The running of the checkfiles command could be tested inside test.py. It is also a good practice to test that the command's --help message (at least) runs using conda/metal.yaml.

You'll find examples of this at:

• https://gitlab.idiap.ch/bob/bob.db.replay/blob/master/bob/db/replay/test.py#L195
• https://gitlab.idiap.ch/bob/bob.db.base/blob/master/conda/meta.yaml#L40