def METHOD_NAME(self, mock_os_dir, mock_command):
    mock_os_dir.return_value = ['foo', 'bar']
    assert self.archive.create_gnu_gzip_compressed('source-dir') \
        == 'foo.tar.gz'
    mock_command.assert_called_once_with(
        [
            'tar', '-C', 'source-dir',
            '--format=gnu', '-cSz', '-f', 'foo.tar.gz', 'bar', 'foo'
        ]
    )