gh-154260: Fix test_flush_parameters on DragonFly BSD (GH-154261)

DragonFly, like FreeBSD, rejects mmap.flush() with MS_ASYNC|MS_INVALIDATE
(EINVAL).  Use str.startswith() so the check also matches DragonFly, whose
sys.platform carries a version suffix.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_mmap.py

@@ -1176,8 +1176,8 @@ def test_flush_parameters(self):
             if hasattr(mmap, 'MS_INVALIDATE'):
                 m.flush(PAGESIZE * 2, flags=mmap.MS_INVALIDATE)
             if hasattr(mmap, 'MS_ASYNC') and hasattr(mmap, 'MS_INVALIDATE'):
-                if sys.platform == 'freebsd':
-                    # FreeBSD doesn't support this combination
+                if sys.platform.startswith(('freebsd', 'dragonfly')):
+                    # FreeBSD and DragonFly don't support this combination
                     with self.assertRaises(OSError) as cm:
                         m.flush(0, PAGESIZE, flags=mmap.MS_ASYNC | mmap.MS_INVALIDATE)
                     self.assertEqual(cm.exception.errno, errno.EINVAL)