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

tests/http/testenv/env.py

@@ -439,6 +439,11 @@ class Env:
     def nghttpx(self) -> Optional[str]:
         return self.CONFIG.nghttpx
 
+    @property
+    def slow_network(self) -> bool:
+        return "CURL_DBG_SOCK_WBLOCK" in os.environ or \
+               "CURL_DBG_SOCK_WPARTIAL" in os.environ
+
     def authority_for(self, domain: str, alpn_proto: Optional[str] = None):
         if alpn_proto is None or \
                 alpn_proto in ['h2', 'http/1.1', 'http/1.0', 'http/0.9']: