gh-130149: cleanup refactorization of test_hmac.py (#131318)

New features:

* refactor `hashlib_helper.requires_hashdigest` in prevision of a future
  `hashlib_helper.requires_builtin_hashdigest` for built-in hashes only
* add `hashlib_helper.requires_openssl_hashdigest` to request OpenSSL
   hashes, assuming that `_hashlib` exists.

Refactoring:

* split hmac.copy() test by implementation
* update how algorithms are discovered for RFC test cases
* simplify how OpenSSL hash digests are requested
* refactor hexdigest tests for RFC test vectors
* typo fix: `assert_hmac_hexdigest_by_new` -> `assert_hmac_hexdigest_by_name`

Improvements:

* strengthen contract on `hmac_new_by_name` and `hmac_digest_by_name`
* rename mixin classes to better match their responsibility

Lib/test/support/hashlib_helper.py

@@ -1,6 +1,7 @@
 import functools
 import hashlib
 import unittest
+from test.support.import_helper import import_module
 
 try:
     import _hashlib
@@ -12,44 +13,81 @@ def requires_hashlib():
     return unittest.skipIf(_hashlib is None, "requires _hashlib")
 
 
-def requires_hashdigest(digestname, openssl=None, usedforsecurity=True):
-    """Decorator raising SkipTest if a hashing algorithm is not available
+def _decorate_func_or_class(func_or_class, decorator_func):
+    if not isinstance(func_or_class, type):
+        return decorator_func(func_or_class)
 
-    The hashing algorithm could be missing or blocked by a strict crypto
-    policy.
+    decorated_class = func_or_class
+    setUpClass = decorated_class.__dict__.get('setUpClass')
+    if setUpClass is None:
+        def setUpClass(cls):
+            super(decorated_class, cls).setUpClass()
+        setUpClass.__qualname__ = decorated_class.__qualname__ + '.setUpClass'
+        setUpClass.__module__ = decorated_class.__module__
+    else:
+        setUpClass = setUpClass.__func__
+    setUpClass = classmethod(decorator_func(setUpClass))
+    decorated_class.setUpClass = setUpClass
+    return decorated_class
+
+
+def requires_hashdigest(digestname, openssl=None, usedforsecurity=True):
+    """Decorator raising SkipTest if a hashing algorithm is not available.
+
+    The hashing algorithm may be missing, blocked by a strict crypto policy,
+    or Python may be configured with `--with-builtin-hashlib-hashes=no`.
 
     If 'openssl' is True, then the decorator checks that OpenSSL provides
-    the algorithm. Otherwise the check falls back to built-in
-    implementations. The usedforsecurity flag is passed to the constructor.
+    the algorithm. Otherwise the check falls back to (optional) built-in
+    HACL* implementations.
 
+    The usedforsecurity flag is passed to the constructor but has no effect
+    on HACL* implementations.
+
+    Examples of exceptions being suppressed:
     ValueError: [digital envelope routines: EVP_DigestInit_ex] disabled for FIPS
     ValueError: unsupported hash type md4
     """
-    def decorator(func_or_class):
-        if isinstance(func_or_class, type):
-            setUpClass = func_or_class.__dict__.get('setUpClass')
-            if setUpClass is None:
-                def setUpClass(cls):
-                    super(func_or_class, cls).setUpClass()
-                setUpClass.__qualname__ = func_or_class.__qualname__ + '.setUpClass'
-                setUpClass.__module__ = func_or_class.__module__
-            else:
-                setUpClass = setUpClass.__func__
-            setUpClass = classmethod(decorator(setUpClass))
-            func_or_class.setUpClass = setUpClass
-            return func_or_class
+    if openssl and _hashlib is not None:
+        def test_availability():
+            _hashlib.new(digestname, usedforsecurity=usedforsecurity)
+    else:
+        def test_availability():
+            hashlib.new(digestname, usedforsecurity=usedforsecurity)
 
-        @functools.wraps(func_or_class)
+    def decorator_func(func):
+        @functools.wraps(func)
         def wrapper(*args, **kwargs):
             try:
-                if openssl and _hashlib is not None:
-                    _hashlib.new(digestname, usedforsecurity=usedforsecurity)
-                else:
-                    hashlib.new(digestname, usedforsecurity=usedforsecurity)
-            except ValueError:
-                raise unittest.SkipTest(
-                    f"hash digest {digestname!r} is not available."
-                )
-            return func_or_class(*args, **kwargs)
+                test_availability()
+            except ValueError as exc:
+                msg = f"missing hash algorithm: {digestname!r}"
+                raise unittest.SkipTest(msg) from exc
+            return func(*args, **kwargs)
         return wrapper
+
+    def decorator(func_or_class):
+        return _decorate_func_or_class(func_or_class, decorator_func)
+    return decorator
+
+
+def requires_openssl_hashdigest(digestname, *, usedforsecurity=True):
+    """Decorator raising SkipTest if an OpenSSL hashing algorithm is missing.
+
+    The hashing algorithm may be missing or blocked by a strict crypto policy.
+    """
+    def decorator_func(func):
+        @requires_hashlib()
+        @functools.wraps(func)
+        def wrapper(*args, **kwargs):
+            try:
+                _hashlib.new(digestname, usedforsecurity=usedforsecurity)
+            except ValueError:
+                msg = f"missing OpenSSL hash algorithm: {digestname!r}"
+                raise unittest.SkipTest(msg)
+            return func(*args, **kwargs)
+        return wrapper
+
+    def decorator(func_or_class):
+        return _decorate_func_or_class(func_or_class, decorator_func)
     return decorator