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>