QCollatorSortKey fails on macOS and Windows

QCollatorSortKey was never unit-tested, and trying to add one, it was found to fail on macOS, where "fail" means that the implementation does not live up to the fundamental property of sort keys, namely that

∀ lhs, rhs ∈ Domain(QString), c ∈ Domain(QCollator):
   sign(c.compare(lhs, rhs)) == sign(c.sortKey(lhs).compare(c.sortKey(rhs))

(where sign := i < 0 ? -1 : i > 0 : 1 : 0).

See https://codereview.qt-project.org/184829 for the failures.