TestSuite: multi-template support in CORRADE_COMPARE_{AS,WITH}().

Now it is possible to write this:

    CORRADE_COMPARE_AS(a, b, std::pair<int, int>);
    CORRADE_COMPARE_AS(a, b, Compare::Container<std::map<int, int>>());

The documentation uses slightly different (GCC-style) syntax (named
varargs parameter) so the arguments are self-descriptive, but the
compiler sees perfectly standard C++.