[BC140] Test for OnOpenPage event

There is a pageextension that changes the "Document Date" to WorkDate() when the page opens. It is working fine - using trigger OnOpenPage().

I'd like to write a test for that:

procedure IsSalesInvoiceDocumentDateWorkday()
    var
        SalesHeader: Record "Sales Header" temporary;
        SalesInvoice: TestPage "Sales Invoice";
    begin
        //Create a dummy record with a document date that is different than the WORKDATE
        SalesHeader.Init();
        SalesHeader.Insert(true);

        SalesHeader."Document Date" := CalcDate('<1D>', WorkDate());
        SalesHeader.Modify();

        SalesInvoice.OpenEdit();
        SalesInvoice.F_DocumentDate.AssertEquals(WorkDate());

I can see that it is a problem which record the "Sales Invoice" will open. But still I am not sure that the pageextension's modification runs with this SalesInvoice.OpenEdit() line.

How can I write a test for this?

Thank you