http2: deflake TestTransportReqBodyAfterResponse_200
http2: deflake TestTransportReqBodyAfterResponse_200

Don't send the END_STREAM flag from the server until the client
sends its END_STREAM. Avoids a flaky failure when the client
sends the END_STREAM in a zero-length DATA frame:

  - The server reads bodySize bytes and half-closes the stream.
  - The client's Response.Body returns EOF.
  - The test calls Response.Body.Close.
  - The client resets the stream.

Avoid hanging forever on the client side of the test if the
server side exits with an error.

Fixes golang/go#48970

Change-Id: Ic921a3f60149abbb5434ec7a53985becea7b23c3
Reviewed-on: https://go-review.googlesource.com/c/net/+/355649
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/net