Use new color testing code for grid test

tests/fakes.vala

@@ -40,6 +40,7 @@ LiveChart.Config create_config(TestContext context = create_context()) {
     var config = new LiveChart.Config();
     config.width = context.surface.get_width();
     config.height = context.surface.get_height();
+
     config.padding = { 0, 0, 0, 0 };
 
     return config;

tests/grid.vala

@@ -1,39 +1,30 @@
 
 private void register_grid() {
 
-    Test.add_func("/LiveChart/Grid#should_generate_padding_when_y_axis_has_not_unit", () => {
+    Test.add_func("/Grid/should_generate_padding_when_y_axis_has_not_unit", () => {
         //given
         var WIDTH = 100;
         var HEIGHT = 100;
 
+        var red = Gdk.RGBA() { red = 1f, green = 0f, blue = 0f, alpha = 1f };
+
         var context = create_context(WIDTH, HEIGHT);
+        context.set_background_color(red);
 
         var config = new LiveChart.Config();
-        config.y_axis.update_bounds(10);
         config.height = HEIGHT;
         config.width = WIDTH;
         config.configure(context.ctx, null);
 
         var grid = new LiveChart.Grid();
-        var background = new LiveChart.Background();
-        background.color = { 1f, 0f, 0f, 1f };
-        background.draw(context.ctx, config);
 
         //when
         grid.draw(context.ctx, config);
-
-        //then
-        var pixbuff = Gdk.pixbuf_get_from_surface(context.surface, 0, 0, WIDTH, HEIGHT);
+        screenshot(context);
 
+        //then
         //Colors between left side and first pixels of "0" char on y-axis
-        var padding_colors = colors_at(pixbuff, WIDTH, HEIGHT)(0, 80, 10, 80);
-        assert(padding_colors.size == 11);
-        foreach (var color in padding_colors) {
-            assert(color.red == 1.0);
-            assert(color.green == 0);
-            assert(color.blue == 0);
-            assert(color.alpha == 1);
-        }
+        assert(has_only_one_color_in_rectangle(context, 0, 80, 10, 80)(red));
     });
 }