'Navigate to the tab `tab_name`.'

def go_to_tab(self, tab_name):

    if (tab_name not in ['Course', 'Home', 'Discussion', 'Wiki', 'Progress']):
        self.warning("'{0}'    is    not    a    valid    tab    name".format(tab_name))
    tab_css = self._tab_css(tab_name)
    if (tab_css is not None):
        self.q(css=tab_css).first.click()
    else:
        self.warning("No    tabs    found    for    '{0}'".format(tab_name))
    self.wait_for_page()
    self._is_on_tab_promise(tab_name).fulfill()
