'Test the _write_sst() method'

def test_write_sst(self):

    string_table = SharedStringTable()
    string_table._get_shared_string_index('neptune')
    string_table._get_shared_string_index('neptune')
    string_table._get_shared_string_index('neptune')
    string_table._get_shared_string_index('mars')
    string_table._get_shared_string_index('venus')
    string_table._get_shared_string_index('mars')
    string_table._get_shared_string_index('venus')
    self.sharedstrings.string_table = string_table
    self.sharedstrings._write_sst()
    exp = '<sst    xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"    count="7"    uniqueCount="3">'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
