BibFormat: setUp/tearDown in unit tests

* BibFormat unit tests were amending bibformat_engine internal
  configuration variable to set up testing path. The implementation
  was lacking however proper tearDown causing all sorts of side
  effects to tests happening afterwards, in particular affecting
  get_record()/print_record() calls for empty record.
  This is now fixed.  (closes #1319)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>