Add argument to multiple_times

[ivancrneto]

class TestOs:
    def test_getcwd(self):
        with stubout(os, "getcwd") as m_getcwd:
            m_getcwd().returns("/mox/path")
            # the second call will return a different value
            m_getcwd().returns("/mox/another/path")
            # the three subsequent calls will return "/"
            # if no argument is passed, multiple_times doesn't limit the number of calls
            m_getcwd().multiple_times(3).returns("/")

        assert os.getcwd() == "/mox/path"
        assert os.getcwd() == "/mox/another/path"
        os.getcwd()
        mox.verify(m_getcwd)

multipe_times need to be able to receive an argument (in the example above: 3), so we expect only three calls. The test above should fail unless we add two new calls to os.getcwd()