Attempt work around for race-y sort of condition in Selenium sizzle checkers.

[jmchilton]

I recently discovered you can add little inline non-functional documentation to source code itself that I believe are called comments or something. I have added one of these things to the code to explain the problem and the work around.

I believe this rare condition explains this failing test here - https://jenkins.galaxyproject.org/view/All/job/selenium/54/testReport/junit/selenium_tests.test_stock_tours/TestStockToursTestCase/test_core_history/. This test has never failed for me locally and this is the only time I've seen it fail on Jenkins as well.

[jmchilton]

The only remaining failures in my latest Jenkins run (post merge of #3450) were these stale errors - https://jenkins.galaxyproject.org/view/All/job/selenium/56/testReport/.

[martenson]

@jmchilton thanks for working on testing!

[jmchilton]

I think that helped - but we still seem to have some problems with this in the tour testing. I'll have to look in depth some more.

======================================================================
ERROR: test_core_history (selenium_tests.test_stock_tours.TestStockToursTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/jenkins/workspace/selenium/test/selenium_tests/framework.py", line 70, in func_wrapper
    return f(self, *args, **kwds)
  File "/var/jenkins/workspace/selenium/test/selenium_tests/test_stock_tours.py", line 33, in test_core_history
    os.path.join(STOCK_TOURS_DIRECTORY, "core.history.yaml"),
  File "/var/jenkins/workspace/selenium/test/galaxy_selenium/navigates_galaxy.py", line 390, in run_tour
    self.run_tour_step(step, i, tour_callback)
  File "/var/jenkins/workspace/selenium/test/galaxy_selenium/navigates_galaxy.py", line 443, in run_tour_step
    element.click()
  File "/var/jenkins/workspace/selenium/.venv/local/lib/python2.7/site-packages/selenium/webdriver/remote/webelement.py", line 77, in click
    self._execute(Command.CLICK_ELEMENT)
  File "/var/jenkins/workspace/selenium/.venv/local/lib/python2.7/site-packages/selenium/webdriver/remote/webelement.py", line 494, in _execute
    return self._parent.execute(command, params)
  File "/var/jenkins/workspace/selenium/.venv/local/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 236, in execute
    self.error_handler.check_response(response)
  File "/var/jenkins/workspace/selenium/.venv/local/lib/python2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 192, in check_response
    raise exception_class(message, screen, stacktrace)
StaleElementReferenceException: Message: stale element reference: element is not attached to the page document
  (Session info: chrome=54.0.2840.71)
  (Driver info: chromedriver=2.25.426924 (649f9b868f6783ec9de71c123212b908bf3b232e),platform=Linux 3.16.0-4-amd64 x86_64) (WARNING: The server did not provide any stacktrace information)