Fix test-roots
Fix test-roots

Better delimit v's lifetime with a local block. This is relevant in
bytecode.

dsheets/ocaml-ctypes,whitequark/ocaml-ctypes,ocamllabs/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,yallop/ocaml-ctypes,whitequark/ocaml-ctypes,yallop/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes