QSignalSpy: fix C'n'P mistake in a qWarning()

The warning for the new-style signal constructor was copied from the
old-style signal constructor, but not adjusted to its new home. The
signal pointer passed here is not the signal "name", but a signal
"pointer" (-to-member-function, but no need to go into that much
detail).

Amends 6fc7d76e7309c01a364b0f72d253735366674f29, but not picking to
very strict LTS branches, just in case someone has a
QTest::ignoreMsg() installed on it.

Pick-to: 6.7 6.6 6.5
Change-Id: Ia1f6b7001f38202ac72f9945c4a822d81562cdbf
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/qsignalspy.h

@@ -63,7 +63,7 @@ public:
             return;
 
         if (!signal0) {
-            qWarning("QSignalSpy: Null signal name is not valid");
+            qWarning("QSignalSpy: Null signal pointer is not valid");
             return;
         }