gh-154289: Fix test_socket.testGetServBy on DragonFly (GH-154290)

DragonFly lists "echo 4/ddp" before "echo 7/tcp" in /etc/services, like
FreeBSD, so avoid the "echo" service there too.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_socket.py

@@ -1302,7 +1302,8 @@ def testGetServBy(self):
         # protocol, at least for modern Linuxes.
         if (
             sys.platform.startswith(
-                ('linux', 'android', 'freebsd', 'netbsd', 'gnukfreebsd'))
+                ('linux', 'android', 'freebsd', 'dragonfly', 'netbsd',
+                 'gnukfreebsd'))
             or is_apple
         ):
             # avoid the 'echo' service on this platform, as there is an