removing xfail on anonymous union test

fixed by compiler-research/CppInterOp#321

Author: Vipul-Cariappa

test/test_datatypes.py

@@ -1558,7 +1558,6 @@ def test30_multi_dim_arrays_of_builtins(test):
                 p = (ctype * len(buf)).from_buffer(buf)
                 assert [p[j] for j in range(width*height)] == [2*j for j in range(width*height)]
 
-    @mark.xfail
     def test31_anonymous_union(self):
         """Anonymous unions place there fields in the parent scope"""