tests: run the half-close case on Windows again

The previous commit lets a half-closed client be written to on Windows
too, so the case left out there is back.

This reverts commit 5d1cdf8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/cases/portmap.py

@@ -1,7 +1,6 @@
 """The port mappers: tcppm forwards a TCP port, udppm a UDP one."""
 
 import socket
-import sys
 import threading
 import time
 
@@ -65,12 +64,6 @@ def run(t):
 
     t.stop_all()
 
-    # WSAPoll reports a hang-up once the peer has half-closed, not once
-    # both sides are shut down as poll() does elsewhere, and sockmap() takes
-    # it for a client it can no longer write to: not covered there yet.
-    if sys.platform == "win32":
-        t.skip("a half-closed client gets the whole answer (not on Windows)")
-    else:
     # A client which shuts down its sending side still gets the whole answer,
     # even when it reads slowly: the server has finished and closed by then,
     # and the rest of the answer waits in the proxy's buffer and the kernel's.