gh-110131: Test argparse's extend action with a non-empty default

TestOptionalsActionAppendWithDefault covers this for 'append'; there was
no equivalent for 'extend', so nothing pinned the behavior the docs now
describe.  The coverage is limited to an option, matching the scope of
the new sentence.

Author: v0ropaev

Lib/test/test_argparse.py

@@ -2470,6 +2470,21 @@ class TestActionExtend(ParserTestCase):
     ]
 
 
+class TestActionExtendWithDefault(ParserTestCase):
+    """Tests the extend action for an Optional with a non-empty default"""
+
+    argument_signatures = [
+        Sig('--baz', action='extend', nargs='+', default=['X']),
+    ]
+    failures = ['a', '--baz', 'a --baz']
+    successes = [
+        ('', NS(baz=['X'])),
+        ('--baz a', NS(baz=['X', 'a'])),
+        ('--baz a b', NS(baz=['X', 'a', 'b'])),
+        ('--baz a --baz b c', NS(baz=['X', 'a', 'b', 'c'])),
+    ]
+
+
 class TestNegativeNumber(ParserTestCase):
     """Test parsing negative numbers"""