tests: fix timing issue in test_group_edit

Occasionally the browser.get() will fetch back the groups list before the
deletion has finished. To avoid that, we wait for the flash message indicating
deletion before moving on.

    Traceback (most recent call last):
      File "/usr/lib/python2.6/site-packages/bkr/inttest/server/selenium/test_group_edit.py", line 394, in test_remove_self_admin_group
        check_group_search_results(b, absent=[Group.by_name(u'admin')])
      File "/usr/lib/python2.6/site-packages/bkr/inttest/server/webdriver_utils.py", line 125, in check_group_search_results
        'not(.//td[1]/a/text()="%s")]' % group.group_name)
      File "/usr/lib64/python2.6/site-packages/selenium/webdriver/remote/webdriver.py", line 230, in find_element_by_xpath
        return self.find_element(by=By.XPATH, value=xpath)
      File "/usr/lib64/python2.6/site-packages/selenium/webdriver/remote/webdriver.py", line 662, in find_element
        {'using': by, 'value': value})['value']
      File "/usr/lib64/python2.6/site-packages/selenium/webdriver/remote/webdriver.py", line 173, in execute
        self.error_handler.check_response(response)
      File "/usr/lib64/python2.6/site-packages/selenium/webdriver/remote/errorhandler.py", line 166, in check_response
        raise exception_class(message, screen, stacktrace)
    NoSuchElementException: Message: Unable to locate element: {"method":"xpath","selector":"//table[@id=\"widget\" and not(.//td[1]/a/text()=\"admin\")]"}

Change-Id: I45647031f56ee45bb1bd330c98c9289c36c93f91