BibFormat: fixes unit tests

* FormatTest failed to append CFG_TMPDIR to sys.path and resulted
  in tests failing under some conditions. (closes #1320)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
BibFormat: checks for bibformat bin

* Before calling call_old_bibformat we check that bibformat
  binary exists. We do not install it anymore so it only exists
  as legacy.

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