`test-helpers` dependencies are not installed

[electrovir]

Dependencies imported within @web/dev-server-core/test-helpers are not included in the package.json for @web/dev-server-core, requiring the user to manually add them. Specifically, I had to install the following dependencies, which I'm not otherwise using:

• portfinder
• chai

[electrovir]

Looks like @web/test-runner-core/test-helpers has the same problem with (at least) its @web/test-runner-mocha dependency.

[bashmish]

This seems like a Good First Issue. Can you make a PR for this?