Before this patch there was no way to reconstruct the invidual chunks
that were sent. All we got was the full resulting body in
`conn.resp_body`.

For completeness, I believe instead of messaging we could store chunks in a list
in the test adapter state and simply append:

    - def send_chunked(state, _status, _headers), do: {:ok, "", %{state | chunks: ""}}
    + def send_chunked(state, _status, _headers), do: {:ok, "", %{state | chunks: []}}

    - def chunk(%{owner: owner, ref: ref, chunks: chunks} = state, chunk) do
    -   send(owner, {ref, :chunk, chunk})
    -   body = chunks <> IO.iodata_to_binary(chunk)
    -   {:ok, body, %{state | chunks: body}}
    - end
    + def chunk(%{owner: owner, ref: ref, chunks: chunks} = state, chunk) do
    +   chunk = IO.iodata_to_binary(chunk)
    +   body = IO.iodata_to_binary([chunks, chunk])
    +   {:ok, body, %{state | chunks: chunks ++ [chunk]}}
    + end

but I was following the existing functions `sent_informs` and `sent_upgrades`.

My use case is to be able to test response streaming using Req :plug
adapter:

    req =
      Req.new(
        plug: fn conn ->
          conn = Plug.Conn.send_chunked(conn, 200)
          {:ok, conn} = Plug.Conn.chunk(conn, "foo")
          {:ok, conn} = Plug.Conn.chunk(conn, "bar")
          conn
        end,
        into: []
      )

    resp = Req.request!(req)
    assert resp.body == ["foo", "bar"]