tst_QStringView: don't go through the QString constructor

Amends d351a97e85.

Pick-to: 6.2 6.5 6.6 6.7
Change-Id: I5201966b308e48989c06fffd17a9aa4d086e6039
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

tests/auto/corelib/text/qstringview/tst_qstringview.cpp

@@ -471,7 +471,7 @@ void tst_QStringView::arg() const
 {
     // nullness checks
     QCOMPARE(QStringView().arg(QStringView()), "");
-    QCOMPARE(QStringView("%1").arg(QStringView()), "");
+    QCOMPARE(QStringView(u"%1").arg(QStringView()), "");
 
 #define CHECK1(pattern, arg1, expected) \
     do { \