[3.14] gh-154279: Skip readline completion tests that fail on DragonFly and illumos (GH-154280) (GH-154288)

They are already skipped on FreeBSD, where a backspace does not remove
the auto-indentation.  The same happens on DragonFly and illumos.

(cherry picked from commit e81f190)

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

Author: serhiy-storchaka

Lib/test/test_pdb.py

@@ -5013,8 +5013,9 @@ def f():
 
         self.assertIn(b'I love Python', output)
 
-    @unittest.skipIf(sys.platform.startswith('freebsd'),
-                     '\\x08 is not interpreted as backspace on FreeBSD')
+    @unittest.skipIf(sys.platform.startswith(('freebsd', 'dragonfly', 'sunos')),
+                     '\\x08 does not remove the auto-indentation with '
+                     'GNU readline on this platform')
     def test_multiline_auto_indent(self):
         script = textwrap.dedent("""
             import pdb; pdb.Pdb().set_trace()
@@ -5053,8 +5054,9 @@ def test_multiline_completion(self):
 
         self.assertIn(b'42', output)
 
-    @unittest.skipIf(sys.platform.startswith('freebsd'),
-                     '\\x08 is not interpreted as backspace on FreeBSD')
+    @unittest.skipIf(sys.platform.startswith(('freebsd', 'dragonfly', 'sunos')),
+                     '\\x08 does not remove the auto-indentation with '
+                     'GNU readline on this platform')
     def test_multiline_indent_completion(self):
         script = textwrap.dedent("""
             import pdb; pdb.Pdb().set_trace()