Suppress output of debug message only produced to test against crash

As a result, also make sure the test will fail if output to debug
stream doesn't produce the expected result.

Change-Id: I9914c9c41c8d8b79f32dfb8e0c735f12e2d59f5e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp

@@ -939,6 +939,7 @@ void tst_QTimeZone::serialize()
 {
     int parts = 0;
 #ifndef QT_NO_DEBUG_STREAM
+    QTest::ignoreMessage(QtDebugMsg, "QTimeZone(\"\")");
     qDebug() << QTimeZone(); // to verify no crash
     parts++;
 #endif