Reenable cross engine read write netCDF test (#2739)

Fixes #2050

I'm not quite sure what was going on, but it passes now.

Author: shoyer

xarray/tests/test_backends.py

@@ -1732,7 +1732,6 @@ def test_engine(self):
         with raises_regex(ValueError, 'can only read'):
             open_dataset(BytesIO(netcdf_bytes), engine='foobar')
 
-    @pytest.mark.xfail(reason='https://github.com/pydata/xarray/issues/2050')
     def test_cross_engine_read_write_netcdf3(self):
         data = create_test_data()
         valid_engines = set()