Stripe billing.test.js should be moved to integrations tests

[travis]

@vasco-santos noted in #320 (comment) that the new billing test I wrote uses the remote Stripe test environment and probably belongs in the integration test suite.

We don't currently do lower-level testing like this in the integration suite, so it will require a bit of work to get the execution context set up properly to enable this, so I'm pulling it out to a separate issue here.