Add a test showing http/2 works

Apollo Server v3 used `node-fetch` `Request`s to represent incoming requests, and those objects aren't compatible with http/2 requests which have colons in header names. See #2333 and #1533.

We think that AS4 (which no longer uses that representation for requests) should support http/2. It would be great to get a PR adding a test!

Start by locating Apollo Server 4's existing HTTP integration tests and the entry point that handles incoming requests. Add coverage for an HTTP/2 request, then run the relevant test suite and confirm the request completes successfully without the header-name incompatibility described in the issue.

Written by the indexing model from the issue text.