[3.15] gh-154416: Fix hanging test_tcflow on DragonFly BSD (GH-154417) (GH-154423)

TCIOFF and TCION transmit STOP and START characters, and closing the
pseudo-terminal waits until they are read.  Discard them.
(cherry picked from commit b0c945a)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_termios.py

@@ -180,6 +180,9 @@ 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):