I could have added something like

    Counts counts{{4, 9, 16}};
    Points points{{0.4, 0.9, 1.1}};
    ws6->setHistogram(3, points, counts);
    for (auto i = 0; i < 5;
         i++) { // modify all 5 spectra in terms of Dx
      ws6->setPointStandardDeviations(i, 3);
      auto dx = Mantid::Kernel::make_cow<HistogramDx>(3, 8.8);
      ws6->setSharedDx(i, dx);
    }

But the current solution seems to be easier since I do not want to check Dx again here; if algorithm executed and did not throw is enough to test (->RunCombinationHelper).

Refs #22218