bpo-40928: notify users running test_decimal on macOS of malloc warnings (GH-26783)

* When trying to allocate very large regions on macOS, malloc does not   fail silently. It sends a noisy error out to STDERR
* This provides a helper function to warn the user, and provides the warning for test_decimal, which consistently generates these warnings on macOS.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/support/__init__.py

@@ -467,6 +467,24 @@ TEST_HOME_DIR = os.path.dirname(TEST_SUPPORT_DIR)
 TEST_DATA_DIR = os.path.join(TEST_HOME_DIR, "data")
 
 
+def darwin_malloc_err_warning(test_name):
+    """Assure user that loud errors generated by macOS libc's malloc are
+    expected."""
+    if sys.platform != 'darwin':
+        return
+
+    import shutil
+    msg = ' NOTICE '
+    detail = (f'{test_name} may generate "malloc can\'t allocate region"\n'
+              'warnings on macOS systems. This behavior is known. Do not\n'
+              'report a bug unless tests are also failing. See bpo-40928.')
+
+    padding, _ = shutil.get_terminal_size()
+    print(msg.center(padding, '-'))
+    print(detail)
+    print('-' * padding)
+
+
 def findfile(filename, subdir=None):
     """Try to find a file on sys.path or in the test directory.  If it is not
     found the argument passed to the function is returned (this does not