gh-154416: Fix hanging test_tcflow on DragonFly BSD

TCIOFF and TCION transmit STOP and START characters, and closing the
pseudo-terminal waits until they are read.  Discard them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_termios.py

@@ -194,6 +194,9 @@ def test_tcflow(self):
             termios.tcflow(self.fd, termios.TCOON)
             termios.tcflow(self.fd, termios.TCIOFF)
             termios.tcflow(self.fd, termios.TCION)
+            # Discard the transmitted STOP and START characters,
+            # otherwise closing the pseudo-terminal can block.
+            termios.tcflush(self.fd, termios.TCOFLUSH)
 
     @support.skip_android_selinux('tcflow')
     def test_tcflow_errors(self):