e2e tests

[ahjohannessen]

As suggested by @fiadliel we should introduce some more tests that cover:

• client, server startup
• e2e single-single request/response
• e2e single-stream request/response
• e2e stream-single request/response
• e2e stream-stream (client closes first) request/response
• e2e stream-stream (server closes first) request/response

There are a lot more possible scenarios (e.g. errors, test against different language implementations), but it's some of the most basic test requirements.