Adds unit test for LoadAsync(string path).

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

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-
+using System.Threading.Tasks;
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.PixelFormats;
@@ -40,6 +40,15 @@ namespace SixLabors.ImageSharp.Tests
                 }
             }
 
+            [Fact]
+            public async Task Path_Agnostic_Async()
+            {
+                using (var img = await Image.LoadAsync(this.Path))
+                {
+                    VerifyDecodedImage(img);
+                }
+            }
+
             [Fact]
             public void Path_Decoder_Specific()
             {