testsuite: better error message

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13631 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
tests/utils: add quotes in Makefile

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13632 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02