gh-66788: Use test.support.subTests in utf-7-imap codec tests

Convert the parametrized utf-7-imap codec tests to the subTests()
decorator.  Add round-trip cases for "+", "+-" (literal, unlike in UTF-7)
and a "," in the Base64 run, and replace the duplicate decode-error cases
with a "&" at the end of the input and an unterminated but otherwise valid
Base64 sequence.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_codecs.py

@@ -1411,46 +1411,45 @@ def test_decode_invalid(self):
     ('&', b'&-'),
     ('&&', b'&-&-'),
     ('A&B', b'A&-B'),
+    # "+" and "+-" are literal, unlike in UTF-7 where "+" is the shift character.
+    ('+', b'+'),
+    ('+-', b'+-'),
     # RFC 3501 section 5.1.3 example.
     ('~peter/mail/台北/日本語',
      b'~peter/mail/&U,BTFw-/&ZeVnLIqe-'),
     # Non-printable ASCII (including TAB) is Base64-encoded, not direct.
     ('a\tb', b'a&AAk-b'),
     ('\x00', b'&AAA-'),
     ('Entw\xfcrfe', b'Entw&APw-rfe'),
+    ('ϰ', b'&A,A-'),                     # "," in the modified Base64 alphabet
     ('☃', b'&JgM-'),                     # snowman
     ('\U0001f600', b'&2D3eAA-'),              # non-BMP (surrogate pair)
     ('Sent &\N{DELETE}', b'Sent &-&AH8-'),
 ]
 
 class Utf7ImapTest(unittest.TestCase):
-    def test_encode(self):
-        for uni, encoded in utf_7_imap_testcases:
-            with self.subTest(uni=uni):
-                self.assertEqual(uni.encode('utf-7-imap'), encoded)
-
-    def test_decode(self):
-        for uni, encoded in utf_7_imap_testcases:
-            with self.subTest(encoded=encoded):
-                self.assertEqual(encoded.decode('utf-7-imap'), uni)
-
-    def test_decode_invalid(self):
-        # position of the first offending byte in each case
-        testcases = [
-            (b'&AAk', 0),           # unterminated shift sequence
-            (b'&Jgg', 0),           # unterminated shift sequence
-            (b'&AB-', 0),           # Base64 length not a multiple of a code unit
-            (b'&@@@-', 0),          # invalid Base64
-            (b'&====-', 0),         # invalid Base64
-            (b'a\x80b', 1),         # 8-bit byte outside a shift sequence
-            (b'a\x1fb', 1),         # control byte outside a shift sequence
-        ]
-        for encoded, start in testcases:
-            with self.subTest(encoded=encoded):
-                with self.assertRaises(UnicodeDecodeError) as cm:
-                    encoded.decode('utf-7-imap')
-                self.assertEqual(cm.exception.encoding, 'utf-7-imap')
-                self.assertEqual(cm.exception.start, start)
+    @support.subTests('uni,encoded', utf_7_imap_testcases)
+    def test_encode(self, uni, encoded):
+        self.assertEqual(uni.encode('utf-7-imap'), encoded)
+
+    @support.subTests('uni,encoded', utf_7_imap_testcases)
+    def test_decode(self, uni, encoded):
+        self.assertEqual(encoded.decode('utf-7-imap'), uni)
+
+    # 'start' is the position of the first offending byte in each case.
+    @support.subTests('encoded,start', [
+        (b'x&', 1),             # "&" just before the end, unterminated
+        (b'&AAAA', 0),          # unterminated, though the Base64 is valid
+        (b'&AB-', 0),           # Base64 length not a multiple of a code unit
+        (b'&@@@-', 0),          # invalid Base64
+        (b'a\x80b', 1),         # 8-bit byte outside a shift sequence
+        (b'a\x1fb', 1),         # control byte outside a shift sequence
+    ])
+    def test_decode_invalid(self, encoded, start):
+        with self.assertRaises(UnicodeDecodeError) as cm:
+            encoded.decode('utf-7-imap')
+        self.assertEqual(cm.exception.encoding, 'utf-7-imap')
+        self.assertEqual(cm.exception.start, start)
 
     def test_encode_lone_surrogate(self):
         with self.assertRaises(UnicodeEncodeError):