Add a test to check the validity of csv download urls

This closes #215

A couple of things to note:

1. The urls for csvs to check are hardcoded into an array at the top of the test.  As this doesn't use Selenium it doesn't need access to a page object, so this seemed like a logical thing to do.

2. The check for `content-length` is using `> 1000` right now. There is likely a better value than that to use. If you can suggest one, @m8ttyB, I will update the pull request accordingly.