Fix missing dll.config file

The file only exists if there are binding redirects.

tests/ImageSharp.Tests/TestUtilities/TestEnvironment.cs

@@ -170,7 +170,10 @@ namespace SixLabors.ImageSharp.Tests
             }
 
             string testProjectConfigPath = TestAssemblyFile.FullName + ".config";
-            File.Copy(testProjectConfigPath, remoteExecutorConfigPath);
+            if (File.Exists(testProjectConfigPath))
+            {
+                File.Copy(testProjectConfigPath, remoteExecutorConfigPath);
+            }
 
             if (Is64BitProcess)
             {