test: migrate highdpi manual test to QRegularExpression

This is part of the migration of qtbase from QRexExp to
QRegularExpression.

Task-number: QTBUG-72587
Change-Id: I0bb1d1409cb0c8f38b7582bb5ce6bbc2b047b6fc
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

tests/manual/highdpi/dragwidget.cpp

@@ -110,7 +110,7 @@ void DragWidget::dropEvent(QDropEvent *event)
 {
     if (event->mimeData()->hasText()) {
         const QMimeData *mime = event->mimeData();
-        QStringList pieces = mime->text().split(QRegExp("\\s+"),
+        QStringList pieces = mime->text().split(QRegularExpression("\\s+"),
                              QString::SkipEmptyParts);
         QPoint position = event->pos();
         QPoint hotSpot;