Prepare testbench

[lucasdicioccio]

I'd like to have a suite for benchmarking and automatically testing http2-client.
We should have:

• at least two server implementations (one of which should provide some PUSH)
  • maybe Warp and Go-HTTP
• at least one other client implementations
  • nghttp

Extra ideas at https://blog.cloudflare.com/tools-for-debugging-testing-and-using-http-2/ .
I'll likely resurrect laborantin-hs for this task.