Don't run markup unit tests in parallel.

See if fixes intermittently failing test.

tests/Avalonia.Markup.UnitTests/Properties/AssemblyInfo.cs

@@ -1,6 +1,7 @@
 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 +35,6 @@ 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)]