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>