Fix __cmp__ tests in sage/combinat/root_system/type_dual.py

[jdemeyer]

The following tests were added to sage/combinat/root_system/type_dual.py by #6588:

    def __cmp__(self, other):
        """
        EXAMPLES::

            sage: B4     = CartanType(['B', 4])
            sage: B4dual = CartanType(['B', 4]).dual()
            sage: F4dual = CartanType(['F', 4]).dual()
            sage: cmp(F4dual, F4dual)
            0
            sage: cmp(F4dual, B4dual)
            1
            sage: cmp(B4dual, F4dual)
            -1
            sage: cmp(B4dual, B4)
            1

        .. todo:: do we really need a cmp, or just eq?
        """

But whether cmp() outputs 1 or -1 is essentially arbitrary. This must be fixed immediately.

[nthiery]

I just made a micro change to check that exchanging F4dual and B4dual inverses the sign. If you are happy with it, you can set a positive review on my behalf.

Thanks for fixing my tests!