Run express server alongside tests

_Do not merge._

Hopefully, this PR is enough to start a conversation.  The goal is to allow integration tests to make use of the awesome api-stubs.  In this PR, the express server is started before invoking Testem.  If it worked (which it doesn't), it would allow tests to access api-stubs in /server/routes/—albeit on a different port than normal.

I think api-stubs should be readily available to tests, either:

* through some integration with the `test` command (preferred), or 
* by documentation instructing the user to fire up the express server in a separate process—some sort of helper command would probably be appropriate here for CI and general convenience