test against the http2 compatibility layer
although we don't support the new HTTP2 APIs, we should run all our tests against both the `http` and `http2` modules, using only the [HTTP/1 API](https://nodejs.org/api/http2.html#http2_compatibility_api)

easiest method would be to set an env var like `KOA_TEST_HTTP_MODULE=http` and run tests with each option. 