[test] rm outdated comment and add bmp and tiff

roottest/root/graf/simpleImages.C

@@ -15,12 +15,10 @@ int simpleImages()
    TH1D h("myHisto", "The Title;the X;the Y", 64, -4, 4);
    h.FillRandom("gaus");
    h.Draw();
-   // We test png and jpeg only. The creation of the gifs 
-   // is affected by an issue on some platforms. See #21561
-   std::vector<std::string> fileNames{"f.jpeg", "f.png", "f.gif"};
+   std::vector<std::string> fileNames{"f.jpeg", "f.png", "f.gif", "f.bmp"}; // TIFF not always there: only ON if asimage_tiff and libtiff installed in system (it's not a builtin like jpeg, png, gif)
    ImagesRAII iraii(fileNames);
    for (auto &&fileName : fileNames) {
       c.SaveAs(fileName.c_str());
    }
    return 0;
-}
+}

roottest/root/graf/simpleImages.ref

@@ -1,4 +1,5 @@
 Info in <TCanvas::Print>: jpg file f.jpeg has been created
 Info in <TCanvas::Print>: png file f.png has been created
 Info in <TCanvas::Print>: gif file f.gif has been created
+Info in <TCanvas::Print>: bmp file f.bmp has been created
 (int) 0