gh-160561: Fix race in test_socket testFromFdError (#160930)

The client thread closes its socket in clientTearDown() concurrently
with the server-side test, so the process-wide fd count could drop
between the two os_helper.fd_count() calls.  Wait for the client thread
to finish before taking the first count.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: asvetlov

Lib/test/test_socket.py

@@ -3231,6 +3231,9 @@ def _testFromFd(self):
     def testFromFdError(self):
         # Make sure that the fromfd() duplicated socket is closed on error.
         fd = self.cli_conn.fileno()
+        # Wait for the client thread to close its socket, otherwise
+        # fd_count() can race with clientTearDown().
+        self.done.wait()
         fd_count_before = os_helper.fd_count()
 
         with self.assertRaises(TypeError):