Fix failures in tst_qdatetime under non-english locales

QDateTime::toString uses the C locale since 5ba66c5622
So don't expect locale specific day- and month-names.

Task-number: QTBUG-80441
Change-Id: I08f53b6b33ed9e7eaaa58df4ca6a966c4ba9ef24
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/time/qdatetime.cpp

@@ -4019,8 +4019,7 @@ QString QDateTime::toString(Qt::DateFormat format) const
     If the datetime is invalid, an empty string will be returned.
 
     \note If localized month and day names are desired, please switch to using
-    QLocale::system().toString() as QDateTime methods shall change to use
-    English (C locale) names at Qt 6.
+    QLocale::system().toString() as QDateTime methods use English (C locale) names.
 
     \sa fromString(), QDate::toString(), QTime::toString(), QLocale::toString()
 */

tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp

@@ -892,8 +892,8 @@ void tst_QDateTime::toString_textDate_data()
     QTest::addColumn<QDateTime>("datetime");
     QTest::addColumn<QString>("expected");
 
-    QString wednesdayJanuary = QLocale::system().dayName(3, QLocale::ShortFormat)
-        + ' ' + QLocale::system().monthName(1, QLocale::ShortFormat);
+    const QString wednesdayJanuary = QLocale::c().dayName(3, QLocale::ShortFormat)
+        + ' ' + QLocale::c().monthName(1, QLocale::ShortFormat);
 
     QTest::newRow("localtime")  << QDateTime(QDate(2013, 1, 2), QTime(1, 2, 3), Qt::LocalTime)
                                 << wednesdayJanuary + QString(" 2 01:02:03 2013");