'Test _xml_formula_element()'

def test_xml_formula_element(self):

    self.writer._xml_formula_element('1+2', 3, [('span', '8')])
    exp = '<c    span="8"><f>1+2</f><v>3</v></c>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
