Fix path to test data

Author: brianpopow

tests/ImageSharp.Tests/Formats/WebP/Vp8ResidualTests.cs

@@ -24,7 +24,7 @@ public void GetResidualCost_Works()
         // arrange
         int ctx0 = 0;
         int expected = 20911;
-        string jsonString = File.ReadAllText(@"TestDataWebp\Vp8Residual.json");
+        string jsonString = File.ReadAllText(Path.Combine("TestDataWebp", "Vp8Residual.json"));
         Vp8Residual residual = JsonSerializer.Deserialize<Vp8Residual>(jsonString);
 
         // act