Add tests for javascript code

Per @ghemawat from https://github.com/google/pprof/pull/790:

None of the javascript in pprof is tested.

One alternative is that we might need to install node in some github action and use that (maybe something like https://github.com/browser-actions/examples).

Another alternative would be to do manually triggered tests: if pprof is invoked with a special flag, it passes that to the javascript, and the javascript code when it sees that flag runs self tests. But it would be better to figure out how to have real tests.