Fix some flaky Selenium tests

[bradenmacdonald]

This is an urgent PR to fix problematic selenium tests with this XBlock, when run on Travis CI.

• Any tests that used AssessmentTestMixin.click_submit() would virtually never pass, because it did not wait for the AJAX call to complete before continuing on to assert follow-up behavior
  • This is by far the biggest problem at the moment
• The whole Selenium test suite now supports both Firefox ( version < 47 ) and Chrome
  • Running tests in a different browser can help with debugging
  • Chrome runs tests locally much faster. To use Chrome locally, run:
    • SELENIUM_BROWSER=chrome run_tests.py
  • Running chrome on Travis CI requires the changes shown in 190eed9
• Replaces deprecated settings path with correct workbench.settings path (hides a warning)
• I included some other fixes for other flaky tests that I've seen; while these changes should not be regressions, I can't say for sure that they've fixed the issue.

I don't believe that this PR fixes all the flaky tests, but it should help.

[bradenmacdonald]

@e-kolpakov can you please review this?

@haikuginger @itsjeyd FYI. Feel free to review too.

[e-kolpakov]

@bradenmacdonald 👍

[bradenmacdonald]

@cahrens would you be able to give this a quick second look please?

[cahrens]

@bradenmacdonald can you review internally? I have a lot of PRs on my plate today (and out next 6 days).

[bradenmacdonald]

@cahrens Yeah I was thinking of doing that anyway but thought I'd ask in case you wanted to hop in. This is just test code anyways. Will do, thanks!

@itsjeyd or @haikuginger can one of you give a second review please?

[haikuginger]

@bradenmacdonald: 👍

Looks good to me. Reading the code, I can definitely see how the changes will produce increased stability.