temporarily disable principality warning test in coercion.ml
    
    Contrarily to the previous commit, this change is *not* completely
    benine: it corresponds to the fact that Jacques' trunk@14523 (a
    principality warning on formats in some situation) has not yet been
    replayed on the format-gadts branch -- I mainly focused on backward
    compatiblity.
    
    The plan is to replay this change really soon, *after* converting
    format6 to a nominal datatype -- this will much simplify the
    re-implementation of the warning in the type-checker.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14847 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02