QSignalSpy::appendArgs(): move-append the QVariantList

Using the rvalue overload of append() is more efficient, since we skip
the alias check and the appendee's atomic ref-count ping-pong inside
lvalue append().

Task-number: QTBUG-123544
Change-Id: Ia76fdf28cba13d524fbbe894658a86a45a1ebe79
Reviewed-by: David Faure <david.faure@kdab.com>
(cherry picked from commit d5d2688acb5b67b41149db3332fa9bad11b39808)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qsignalspy.h

@@ -200,7 +200,7 @@ private:
             else
                 list << QVariant(QMetaType(type), a[i + 1]);
         }
-        append(list);
+        append(std::move(list));
 
         if (m_waiting) {
             locker.unlock();