QCollatorSortKey is not working properly without ICU support

While extending tst_QCollator to support QCollatorSortKey, I've noticed that on platforms without ICU (!QT_CONFIG(icu)) the result of QCollatorSortKey::compare is not always equal to the results of QCollator::compare.

Sample code to illustrate the problem

auto asSign = [](int compared) { 
    return compared < 0 ? -1 : compared > 0 ? 1 : 0;
};

QString str1("some str");
QString str2("some other str");

QCollator collator(/*some initialization params*/);

int collatorResult = asSign(collator.compare(str1, str2));

QCollatorSortKey key1 = collator.sortKey(str1);
QCollatorSortKey key2 = collator.sortKey(str2);

int collatorKeyResult = asSign(key1.compare(key2));

Based on the parameters of collator (case sensitivity, numeric mode, ignore punctuation) the results of the 2 comparisons can be equal or different, which is quite confusing for me.

A real code for the test can be found in qtbase/tests/auto/corelib/text/qcollator/tst_qcollator.cpp

Good starting points are qtbase/src/corelib/text/qcollator_macx.cpp (for Mac) and qtbase/src/corelib/text/qcollator_win.cpp (for Windows)