Construct the test bitmaps directly in wxImageList test code

Don't use external image files, this makes it simpler to run the test
(it can be now done from any directory) and also experiment with it
(e.g. by making the test image fully transparent or fully opaque).

Remove the now unneeded .ico file but keep the .png one still used by
another test.

build/cmake/tests/gui/CMakeLists.txt

@@ -182,7 +182,6 @@ set(TEST_GUI_DATA
     image/paste_result_background_plus_overlay_transparent_border_semitransparent_square.png
     image/paste_result_no_background_square_over_circle.png
     image/wx.png
-    image/wx.ico
     image/toucan.png
     image/toucan_hue_0.538.png
     image/toucan_sat_-0.41.png

tests/graphics/imagelist.cpp

@@ -68,8 +68,6 @@ ImageListTestCase::ImageListTestCase()
                  : bmpRGB(32, 32, 24),
                    bmpMask(32, 32, 1)
 {
-    wxInitAllImageHandlers();
-
     {
         wxMemoryDC mdc(bmpRGB);
         mdc.SetBackground(*wxBLUE_BRUSH);
@@ -79,7 +77,15 @@ ImageListTestCase::ImageListTestCase()
     }
     REQUIRE(bmpRGB.IsOk());
 
-    bmpRGBA.LoadFile("image/wx.png", wxBITMAP_TYPE_PNG);
+    // Make a bitmap with some transparent and semi-transparent pixels.
+    wxImage imgWithAlpha(32, 32);
+    imgWithAlpha.SetAlpha();
+    unsigned char* const alpha = imgWithAlpha.GetAlpha();
+    for ( unsigned char* a = alpha; a < alpha + 32*32; ++a )
+        *a = wxALPHA_OPAQUE;
+    alpha[0] = wxALPHA_TRANSPARENT;
+    alpha[1] = wxALPHA_OPAQUE / 2;
+    bmpRGBA = wxBitmap(imgWithAlpha);
     REQUIRE(bmpRGBA.IsOk());
 
     {
@@ -103,7 +109,7 @@ ImageListTestCase::ImageListTestCase()
     bmpRGBAWithMask.SetMask(new wxMask(bmpMask));
     REQUIRE(bmpRGBAWithMask.IsOk());
 
-    ico.LoadFile("image/wx.ico", wxBITMAP_TYPE_ICO);
+    ico.CopyFromBitmap(bmpRGBWithMask);
     REQUIRE(ico.IsOk());
 
     REQUIRE(bmpRGB.HasAlpha() == false);