bpo-40275: Move requires_hashdigest() to test.support.hashlib_helper (GH-19716)

Add a new test.support.hashlib_helper submodule.

Lib/test/support/hashlib_helper.py

@@ -0,0 +1,38 @@
+import functools
+import hashlib
+import unittest
+
+try:
+    import _hashlib
+except ImportError:
+    _hashlib = None
+
+
+def requires_hashdigest(digestname, openssl=None, usedforsecurity=True):
+    """Decorator raising SkipTest if a hashing algorithm is not available
+
+    The hashing algorithm could be missing or blocked by a strict crypto
+    policy.
+
+    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.
+
+    ValueError: [digital envelope routines: EVP_DigestInit_ex] disabled for FIPS
+    ValueError: unsupported hash type md4
+    """
+    def decorator(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}' is not available."
+                )
+            return func(*args, **kwargs)
+        return wrapper
+    return decorator