[PreviewPane] Porting .NET Core 3.1 (#8432)

* PreviewPane porting .NET core 3.1

* Adding in misspellings

Co-authored-by: Clint Rutkas <clint@rutkas.com>

src/modules/previewpane/STATestClassAttribute.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+// Used for STA tests in PreviewPane
+namespace Microsoft.PowerToys.STATestExtension
+{
+    public class STATestClassAttribute : TestClassAttribute
+    {
+        public override TestMethodAttribute GetTestMethodAttribute(TestMethodAttribute testMethodAttribute)
+        {
+            if (testMethodAttribute is STATestMethodAttribute)
+            {
+                return testMethodAttribute;
+            }
+
+            return new STATestMethodAttribute(base.GetTestMethodAttribute(testMethodAttribute));
+        }
+    }
+}