gh-154863: Check the encoded bytes in the shift state flush test

The test decoded the output back and checked the ASCII in the result, so
it tested the decoder too. The ASCII is written to the output as is, so
checking it there tests only the encoder. Comparing with the same text
without the character also detects a character encoded as nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_codecs.py

@@ -3835,9 +3835,9 @@ def test_encode_shift_state_flush(self):
         # ISO-2022-CN-EXT).  That must not be read as a substituted character:
         # doing so discarded the whole output, ASCII included.
         #
-        # Only the ASCII around the character is checked, not a full round-trip.
-        # An iconv that cannot represent the character either rejects it or
-        # substitutes for it silently, as macOS does for ISO-2022-CN.
+        # Only the ASCII around the character is checked, in the encoded bytes:
+        # it is written there as is.  An iconv that cannot represent the
+        # character rejects it or substitutes for it silently.
         tested = False
         for enc, text in _ICONV_SHIFT_STATE:
             if not iconv_encoding_available(enc):
@@ -3848,10 +3848,10 @@ def test_encode_shift_state_flush(self):
                 except UnicodeEncodeError:
                     continue
                 tested = True
-                self.assertNotEqual(data, b'')
-                decoded = codecs.iconv_decode(enc, data, 'strict', True)[0]
-                self.assertStartsWith(decoded, 'ABC')
-                self.assertEndsWith(decoded, 'DEF')
+                self.assertIn(b'ABC', data)
+                self.assertIn(b'DEF', data)
+                # Something was written for the character itself.
+                self.assertNotEqual(data, codecs.iconv_encode(enc, 'ABCDEF')[0])
         if not tested:
             self.skipTest('no shift-state iconv encoding is available')