PR#6010: Big_int.extract_big_int gives wrong results on negative arguments


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13660 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6010 continued: more tests.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13661 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02