Adds unit test for LoadAsync(Configuration configuration, string path, IImageDecoder decoder).

tests/ImageSharp.Tests/Image/ImageTests.Load_FileSystemPath_UseDefaultConfiguration.cs

@@ -67,6 +67,15 @@ namespace SixLabors.ImageSharp.Tests
                 }
             }
 
+            [Fact]
+            public async Task Path_Decoder_Agnostic_Async()
+            {
+                using (var img = await Image.LoadAsync(Configuration.Default, this.Path, new BmpDecoder()))
+                {
+                    VerifyDecodedImage(img);
+                }
+            }
+
             [Fact]
             public void Path_OutFormat_Specific()
             {