gh-155430: fix hmac.digest bigmem test (GH-155431)

(cherry picked from commit 948fd7e)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: picnixz

Lib/test/test_hmac.py

@@ -1452,19 +1452,25 @@ class OperatorCompareDigestTestCase(CompareDigestMixin, unittest.TestCase):
 class PyMiscellaneousTests(unittest.TestCase):
     """Miscellaneous tests for the pure Python HMAC module."""
 
+    @staticmethod
+    def mock__compute_digest_fallback(hmac):
+        fn = getattr(hmac, meth := "_compute_digest_fallback")
+        return patch.object(hmac, meth, wraps=fn)
+
+    @staticmethod
+    def mock_HMAC_method(hmac, method):
+        fn = getattr(hmac.HMAC, method)
+        return patch.object(hmac.HMAC, method, autospec=True, wraps=fn)
+
     @hashlib_helper.requires_builtin_hmac()
     def test_hmac_constructor_uses_builtin(self):
         # Block the OpenSSL implementation and check that
         # HMAC() uses the built-in implementation instead.
         hmac = import_fresh_module("hmac", blocked=["_hashlib"])
 
-        def watch_method(cls, name):
-            wraps = getattr(cls, name)
-            return patch.object(cls, name, autospec=True, wraps=wraps)
-
         with (
-            watch_method(hmac.HMAC, '_init_openssl_hmac') as f,
-            watch_method(hmac.HMAC, '_init_builtin_hmac') as g,
+            self.mock_HMAC_method(hmac, '_init_openssl_hmac') as f,
+            self.mock_HMAC_method(hmac, '_init_builtin_hmac') as g,
         ):
             _ = hmac.HMAC(b'key', b'msg', digestmod="sha256")
             f.assert_not_called()
@@ -1535,11 +1541,11 @@ def do_test_hmac_digest_overflow_error_switch_to_slow(self, hmac, size):
         bigkey = b'K' * size
         bigmsg = b'M' * size
 
-        with patch.object(hmac, "_compute_digest_fallback") as slow:
+        with self.mock__compute_digest_fallback(hmac) as slow:
             hmac.digest(bigkey, b'm', "md5")
             slow.assert_called_once()
 
-        with patch.object(hmac, "_compute_digest_fallback") as slow:
+        with self.mock__compute_digest_fallback(hmac) as slow:
             hmac.digest(b'k', bigmsg, "md5")
             slow.assert_called_once()
 
@@ -1550,7 +1556,7 @@ def test_hmac_digest_no_overflow_error_in_fallback(self, size):
 
         for key, msg in [(b'K' * size, b'm'), (b'k', b'M' * size)]:
             with self.subTest(keysize=len(key), msgsize=len(msg)):
-                with patch.object(hmac, "_compute_digest_fallback") as slow:
+                with self.mock__compute_digest_fallback(hmac) as slow:
                     hmac.digest(key, msg, "md5")
                     slow.assert_called_once()