gh-107862: Add expected failure for subjects that look like encoded words

A header value that happens to be a well-formed RFC 2047 encoded word
is written out verbatim, so reparsing the message decodes it:
folding 'Subject' '=?utf-8?q?x?=' comes back as 'x'. As far as I can
tell RFC 2047 section 5 requires an encoder to encode such text.

This records the behavior as an expected failure rather than keeping
the round trip property away from it, so the test starts failing when
the generator is fixed.

Author: pganssle

Lib/test/test_email/test_policy.py

@@ -308,7 +308,8 @@ def test_wide_char_too_long_for_line_does_not_cause_inf_loop(self):
     # Pieces chosen to exercise folding: words, runs of whitespace, RFC 5322
     # specials, characters needing 2, 3 and 4 byte UTF-8 encodings, and the
     # delimiters of RFC 2047 encoded words.  The pieces cannot combine into a
-    # well-formed encoded word (there is no lone '?').
+    # well-formed encoded word (there is no lone '?'); that case is a known
+    # failure covered by test_fold_encoded_word_lookalike_round_trip.
     _fold_pieces = hypothesis.strategies.sampled_from([
         'a', 'word', 'longwordlongwordlongword', ' ', '  ', '\t', ',', ';',
         ':', '(', ')', '<', '>', '"', '@', '\\', '.', '-', '_', '\xa0',
@@ -355,6 +356,25 @@ def test_fold_unstructured_round_trip(self, subject, maxlen):
         if subject == subject.lstrip(' \t'):
             self.assertEqual(reparsed, subject)
 
+    @unittest.expectedFailure
+    @hypothesis.given(
+        word=hypothesis.strategies.text(alphabet='abc', min_size=1, max_size=5),
+        trailer=hypothesis.strategies.sampled_from(['', ')', ' word']),
+    )
+    @hypothesis.example(word='x', trailer='')
+    @hypothesis.example(word='x', trailer=')')
+    def test_fold_encoded_word_lookalike_round_trip(self, word, trailer):
+        # Text that happens to be a well-formed RFC 2047 encoded word is
+        # written out verbatim, so it is decoded when the message is
+        # reparsed.  As far as I can tell RFC 2047 section 5 requires an
+        # encoder to encode such text; until that is done this is a known
+        # failure.
+        subject = f'=?utf-8?q?{word}?={trailer}'
+        folded = email.policy.default.fold('Subject', subject)
+        reparsed = email.message_from_string(
+            folded, policy=email.policy.default)['Subject']
+        self.assertEqual(reparsed, subject)
+
     def test_verify_generated_headers(self):
         # Turning protection off allows header injection
         policy = email.policy.default.clone(verify_generated_headers=False)