convert test_str tests to fort_unit
convert test_str tests to fort_unit

these are failing currently as expected

Drup/ocaml-re,dsheets/ocaml-re,dsheets/ocaml-re,bfops/ocaml-re,Drup/ocaml-re,bfops/ocaml-re,bfops/ocaml-re,dsheets/ocaml-re