Fix intermittent test failures.

tests/Perspex.SceneGraph.UnitTests/Properties/AssemblyInfo.cs

@@ -2,5 +2,9 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System.Reflection;
+using Xunit;
 
 [assembly: AssemblyTitle("Perspex.SceneGraph.UnitTests")]
+
+// Don't run tests in parallel.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]