Selenium tests for stbt batch report scripts

While I was working on https://github.com/drothlis/stb-tester/pull/123 I always felt insecure because of the lack of tests for `instaweb` and `edit-testrun.js` that can catch regression errors, and I had to depend on my ad-hoc manual tests. This pull request adds three tests to verify the dynamic behaviour of stbt batch report pages.