tst_collections.cpp: fix RecursiveList comparison case

The RecursiveList structure was defined like this:

 struct RecursiveList : public QList<RecursiveList> {};

However, this definition does not make any sense when it comes to the
relational operators. QList<T> needs to have operator<() defined for
the contained type T.

The current implementation of QTypeTraits::compare_lt_result_container
simply enables operator<() if Container is a base type of the contained
type. This unblocks the compilation of checks like

 static_assert(QTypeTraits::has_operator_less_than_v<RecursiveList>);

However, this is useless in practice.

This commit updates the struct to have a meaningful definition of
operator<().

Amends 9f13842fe6.

Task-number: QTBUG-120305
Pick-to: 6.8 6.5
Change-Id: Iaee96385a33ff131bdceabe945265b3285a370c2
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

tests/auto/corelib/tools/collections/tst_collections.cpp

@@ -155,7 +155,15 @@ struct Dummy
     bool operator<(const Dummy &) const { return false; }
 };
 
-struct RecursiveList : public QList<RecursiveList> {};
+struct RecursiveList : public QList<RecursiveList>
+{
+    friend bool operator<(const RecursiveList &lhs, const RecursiveList &rhs)
+    {
+        using Base = QList<RecursiveList>;
+        // compare some non-QList members here
+        return static_cast<const Base &>(lhs) < static_cast<const Base &>(rhs);
+    }
+};
 struct RecursiveSet : public QSet<RecursiveSet> {};
 struct RecursiveMapV : public QMap<Dummy, RecursiveMapV> {};
 struct RecursiveMapK : public QMap<RecursiveMapK, Dummy> {};