Fix HTML pagination test for high DPI values

The standard margins, expressed in millimeters, could result in the
usable page space being much smaller than 1000px used for the DC size
when using higher DPIs, which means that the test checking that a 2400px
image took only 3 pages could fail, as it could require 4 of them in
this case.

Fix this by getting rid of the margins, as this should ensure that the
page height is exactly 1000px now, independently of the actual DPI.

tests/html/htmprint.cpp

@@ -51,6 +51,14 @@ TEST_CASE("wxHtmlPrintout::Pagination", "[html][print]")
 {
     wxHtmlPrintout pr;
 
+    // Pagination works in terms of printer page size, which is obtained by
+    // subtracting margins from the total size and margins are expressed in
+    // millimeters, so their conversion to pixels depends on DPI. To ensure
+    // that we get the same results for all values of DPI, get just get rid
+    // of the margins entirely (it would also be possible to adjust them by
+    // the DPI-dependent factor, but it doesn't seem to be worth doing it).
+    pr.SetMargins(0, 0, 0, 0, 0);
+
     wxBitmap bmp(1000, 1000);
     wxMemoryDC dc(bmp);
     pr.SetUp(dc);