improve backtrace testsuite
Test the behavior of the deprecated primitive [caml_get_exception_backtrace],
and minimal tests for hashing/comparison of raw backtrace slots.
git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14782 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02