tweak tests/backtrace to also test ocamlopt
    
    Note that native and byte-test share the same reference file, so in
    particular they check that ocamlc and ocamlopt report uncaught
    exception in the exact same way. Passing this test required a change
    in asmrun/backtrace.c/print_location.
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13816 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02