fix(test): Remove cache busting URLs in dev mode.

* If `static_max_age` is set in `local.json`, .JS files are served with a maxAge of 0 already. ETags will still be checked.
* Enables the use of browser dev tools. With dynamic cache busting URLs, filenames would change from one run to the next.

fixes #1145
Merge pull request #1275 from mozilla/issue-1145-no-cache-in-dev

fix(test): Remove cache busting URLs in dev mode.