TestSuite: method for comparing sorted containers usable by humans.

Previously it was ultra-annoying to compare sorted containers, e.g.:

    CORRADE_COMPARE_WITH(a, b, TestSuite::Compare::Container<std::vector
        <std::string>>(TestSuite::Compare::ContainerMethod::Sorted));

Moreover it wasn't possible to compare containers with type not having
comparison operators (i.e. operator<()). As there are only two options
(comparing unsorted vs. comparing sorted), the latter is now implemented
using SortedContainer pseudo-type, resulting in this awesomely intuitive
code:

    CORRADE_COMPARE_AS(a, b, TestSuite::Compare::SortedContainer);

Don't know where I forgot my brain when implementing this for the first
time.