tst_qfont: clear style name in test font

The test failed if qApp->font() had a styleName() set,
when testing old serialization formats which didn't serialize it.

Change-Id: If0236d354be144b3a990e074a22f796fffb1ed18
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Konstantin Shegunov <kshegunov@gmail.com>

tests/auto/gui/text/qfont/tst_qfont.cpp

@@ -358,6 +358,8 @@ void tst_QFont::serialize_data()
     // Versions <= Qt 2.1 had broken point size serialization,
     // so we set an integer point size.
     basicFont.setPointSize(9);
+    // Versions <= Qt 5.4 didn't serialize styleName, so clear it
+    basicFont.setStyleName(QString());
 
     QFont font = basicFont;
     QTest::newRow("defaultConstructed") << font << QDataStream::Qt_1_0;