remove xfail from test_cross_engine_read_write_netcdf4 (pydata#2741)

Author: Joe Hamman

xarray/tests/test_backends.py

@@ -1800,7 +1800,6 @@ def test_complex(self):
             with self.roundtrip(expected) as actual:
                 assert_equal(expected, actual)
 
-    @pytest.mark.xfail(reason='https://github.com/pydata/xarray/issues/535')
     def test_cross_engine_read_write_netcdf4(self):
         # Drop dim3, because its labels include strings. These appear to be
         # not properly read with python-netCDF4, which converts them into