Disable test parallelization in PowerShellEditorServices.Test project

More test instability, I think xUnit's parallelization must have become
more aggressive in 2.2.

Author: daviwil

test/PowerShellEditorServices.Test/AssemblyInfo.cs

@@ -0,0 +1,9 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Xunit;
+
+// Disable test parallelization to avoid port reuse issues
+[assembly: CollectionBehavior(DisableTestParallelization = true)]