Fix unit tests

We had a BUG!

src/ImageSharp/PixelFormats/Alpha8.cs

@@ -114,7 +114,10 @@ namespace SixLabors.ImageSharp.PixelFormats
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void ToRgba32(ref Rgba32 dest)
         {
-            dest.PackedValue = this.PackedValue;
+            dest.R = 0;
+            dest.G = 0;
+            dest.B = 0;
+            dest.A = this.PackedValue;
         }
 
         /// <inheritdoc />

tests/ImageSharp.Tests/PixelFormats/PackedPixelTests.cs

@@ -106,7 +106,7 @@ namespace SixLabors.ImageSharp.Tests.Colors
             float z = +0.5f;
             float w = -0.7f;
             var argb = new Argb32(x, y, z, w);
-            Assert.Equal(0x001a0080u, argb.PackedValue);
+            Assert.Equal(0x80001a00u, argb.PackedValue);
 
             // Test ordering
             var rgb = default(Rgb24);