[tests] [qt] Introduce qt/test/util with a generalized ConfirmMessage

ConfirmMessage is reused in future tests apart from its single usage here.

src/qt/test/util.cpp

@@ -0,0 +1,22 @@
+#include <qt/callback.h>
+
+#include <QApplication>
+#include <QMessageBox>
+#include <QTimer>
+#include <QString>
+#include <QPushButton>
+#include <QWidget>
+
+void ConfirmMessage(QString* text, int msec)
+{
+    QTimer::singleShot(msec, makeCallback([text](Callback* callback) {
+        for (QWidget* widget : QApplication::topLevelWidgets()) {
+            if (widget->inherits("QMessageBox")) {
+                QMessageBox* messageBox = qobject_cast<QMessageBox*>(widget);
+                if (text) *text = messageBox->text();
+                messageBox->defaultButton()->click();
+            }
+        }
+        delete callback;
+    }), SLOT(call()));
+}