fix broken tests

tests/ImageSharp.Tests/TestUtilities/ImageProviders/BlankProvider.cs

@@ -11,7 +11,7 @@ namespace ImageSharp.Tests
     public abstract partial class TestImageProvider<TColor>
         where TColor : struct, IPixel<TColor>
     {
-        private class BlankProvider : TestImageProvider<TColor>
+        private class BlankProvider : TestImageProvider<TColor>, IXunitSerializable
         {
             public BlankProvider(int width, int height)
             {

tests/ImageSharp.Tests/TestUtilities/ImageProviders/FileProvider.cs

@@ -55,14 +55,14 @@ namespace ImageSharp.Tests
                 return this.Factory.CreateImage(cachedImage);
             }
 
-            public void Deserialize(IXunitSerializationInfo info)
+            public override void Deserialize(IXunitSerializationInfo info)
             {
                 this.filePath = info.GetValue<string>("path");
 
                 base.Deserialize(info); // must be called last
             }
 
-            public void Serialize(IXunitSerializationInfo info)
+            public override void Serialize(IXunitSerializationInfo info)
             {
                 base.Serialize(info);
                 info.AddValue("path", this.filePath);

tests/ImageSharp.Tests/TestUtilities/ImageProviders/TestImageProvider.cs

@@ -13,7 +13,7 @@ namespace ImageSharp.Tests
     /// Provides <see cref="Image{TColor}" /> instances for parametric unit tests.
     /// </summary>
     /// <typeparam name="TColor">The pixel format of the image</typeparam>
-    public abstract partial class TestImageProvider<TColor> : IXunitSerializable
+    public abstract partial class TestImageProvider<TColor> 
         where TColor : struct, IPixel<TColor>
     {
         public PixelTypes PixelType { get; private set; } = typeof(TColor).GetPixelType();