chore: add tests

[phryneas]

This adds

• snapshot tests for the extended schema
• queries against schema & db against _entites with
  • nodeId
  • a single primary key
  • different types via single primary key
  • a combined primary key
• an integration test that starts postgraphile, a federated service and an apollo gateway and
  • snapshot-tests the generated schema
  • fires a query and validates that an additional property is added by the federated service

As this PR includes #3 (it wouldn't make sense to me to not test that while I'm at it), the diff is a bit messy.
You can get a cleaner diff here:
https://github.com/phryneas/federation/compare/pr-3...phryneas:pr-5?expand=1

[phryneas]

And it's running in Travis. Gonna head home :)

Oh, once more: this was done in @mayflower slack time. If there's gonna be any Release Notes I'd be happy for a mention :)

[benjie]

I'd like to see this split into two PRs: one that only adds tests for the existing functionality (so that we can be sure that nothing has been broken); and then a second PR that adds the code changes included (and relevant additional tests).

[phryneas]

Sure, I'll do that within the next few days.

[phryneas]

@benjie okay, tests for #3 are removed, this now just tests the current master.
I'll add those back to #3 once this is merged :)

[benjie]

LGTM 👍