http2: polish things around POST

http2: polish things around POST

- added test cases for various code paths
- fixed handling of blocked write when stream had
  been closed inbetween attempts
- re-enabled DEBUGASSERT on send with smaller data size

- in debug builds, environment variables can be set to simulate a slow
  network when sending data. cf-socket.c and vquic.c support
  * CURL_DBG_SOCK_WBLOCK: percentage of send() calls that should be
    answered with a EAGAIN. TCP/UNIX sockets.
    This is chosen randomly.
  * CURL_DBG_SOCK_WPARTIAL: percentage of data that shall be written
    to the network. TCP/UNIX sockets.
    Example: 80 means a send with 1000 bytes would only send 800
    This is applied to every send.
  * CURL_DBG_QUIC_WBLOCK: percentage of send() calls that should be
    answered with EAGAIN. QUIC only.
    This is chosen randomly.

Closes #11756