tbb-testsuite: fix the play_videos test

The play_videos test currently fails with:

Traceback (most recent call last):
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/marionette_test/testcases.py", line 156, in run
testMethod()
File "/home/tbb-testsuite/tbb-testsuite/marionette/tor_browser_tests/test_page.py", line 32, in test_page
elt = self.marionette.find_element('id', 'test_result');
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 1852, in find_element
return self._send_message("findElement", body, key="value")
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/decorators.py", line 23, in _
return func(*args, **kwargs)
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 740, in _send_message
    self._handle_error(err)
  File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 764, in _handle_error
    raise errors.lookup(error)(message, stacktrace=stacktrace)
NoSuchElementException: Unable to locate element: test_result
stacktrace:
	WebDriverError@chrome://marionette/content/error.js:178:5
	NoSuchElementError@chrome://marionette/content/error.js:434:5
element.find/</<@chrome://marionette/content/element.js:341:16