In-browser end-to-end integration tests

[divmain]

Sanity check that changes to build process don't destroy anything when run in supported browsers.

[divmain]

We can test more deeply and thoroughly by relying on jsdom for functional tests. That way, we have direct access to both the build environment and the run-time environment from within a single test. Much of this is already wrapped up into interlock-test-helper.

[divmain]

We're already doing a lot of this in the plugins repo, so porting over the functional test mechanisms from there should suffice to close this issue.