In case anyone else needs this, I wrote a method to do the request-response cycle on a handler for use in minitest.cr:
```
  def app
    @app ||= App.new
  end

  def request(method, path, headers = nil, body = nil)
    req = HTTP::Request.new "GET", "/urls"
    io = IO::Memory.new
    resp = HTTP::Server::Response.new(io)
    context = HTTP::Server::Context.new(req, resp)
    app.call(context)
    resp.close
    io.rewind
    HTTP::Client::Response.from_io(io).not_nil!
  end
```