#542: reduce test output image sizes

test images don't have to be that big for axial gradients.
It's sufficient to show they're constant across the axis and correct along the axis.

tests/ImageSharp.Tests/Drawing/FillLinearGradientBrushTests.cs

@@ -18,12 +18,12 @@ namespace SixLabors.ImageSharp.Tests.Drawing
         public void LinearGradientBrushWithEqualColorsReturnsUnicolorImage()
         {
             string path = TestEnvironment.CreateOutputDirectory("Fill", "LinearGradientBrush");
-            using (var image = new Image<Rgba32>(500, 500))
+            using (var image = new Image<Rgba32>(10, 10))
             {
                 LinearGradientBrush<Rgba32> unicolorLinearGradientBrush = 
                     new LinearGradientBrush<Rgba32>(
                         new SixLabors.Primitives.Point(0, 0),
-                        new SixLabors.Primitives.Point(500, 0),
+                        new SixLabors.Primitives.Point(10, 0),
                         new LinearGradientBrush<Rgba32>.ColorStop(0, Rgba32.Red),
                         new LinearGradientBrush<Rgba32>.ColorStop(1, Rgba32.Red));
                 
@@ -34,8 +34,8 @@ namespace SixLabors.ImageSharp.Tests.Drawing
                 {
                     Assert.Equal(Rgba32.Red, sourcePixels[0, 0]);
                     Assert.Equal(Rgba32.Red, sourcePixels[9, 9]);
-                    Assert.Equal(Rgba32.Red, sourcePixels[199, 149]);
+                    Assert.Equal(Rgba32.Red, sourcePixels[5, 5]);
-                    Assert.Equal(Rgba32.Red, sourcePixels[499, 499]);
+                    Assert.Equal(Rgba32.Red, sourcePixels[3, 8]);
                 }
             }
         }
@@ -44,9 +44,8 @@ namespace SixLabors.ImageSharp.Tests.Drawing
         public void HorizontalLinearGradientBrushReturnsUnicolorColumns()
         {
             int width = 500;
-            int height = 500;
+            int height = 10;
             int lastColumnIndex = width - 1;
-            int lastRowIndex = height - 1;
             
             
             string path = TestEnvironment.CreateOutputDirectory("Fill", "LinearGradientBrush");
@@ -60,7 +59,7 @@ namespace SixLabors.ImageSharp.Tests.Drawing
                         new LinearGradientBrush<Rgba32>.ColorStop(1, Rgba32.Yellow));
                 
                 image.Mutate(x => x.Fill(unicolorLinearGradientBrush));
-                image.Save($"{path}/horizontalGradient.png");
+                image.Save($"{path}/horizontalRedToYellow.png");
 
                 using (PixelAccessor<Rgba32> sourcePixels = image.Lock())
                 {