jsonalchemy: unit tests for module import fix

• Adds a field to fields.cfg, which is a name of module.

• Adds a test that will try to convert a marcxml to json and then from json to marcxml. In this test email is a field which is not present in the marcxml, and then bibfield will try to import it like a module.

Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>