Don't run render tests in parallel.

Tests/Perspex.RenderTests/Properties/AssemblyInfo.cs

@@ -1,6 +1,6 @@
 using System.Reflection;
-using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using Xunit;
 
 // General Information about an assembly is controlled through the following
 // set of attributes. Change these attribute values to modify the information
@@ -34,3 +34,6 @@ using System.Runtime.InteropServices;
 // [assembly: AssemblyVersion("1.0.*")]
 [assembly: AssemblyVersion("1.0.0.0")]
 [assembly: AssemblyFileVersion("1.0.0.0")]
+
+// Don't run tests in parallel.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]