[3.9] bpo-44852: Support ignoring specific DeprecationWarnings wholesale in regrtest (GH-27634) (GH-27785)

(cherry picked from commit a0a6d39295)

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

Lib/test/test_support.py

@@ -11,6 +11,8 @@ import tempfile
 import textwrap
 import time
 import unittest
+import warnings
+
 from test import support
 from test.support import script_helper
 from test.support import socket_helper
@@ -19,6 +21,33 @@ TESTFN = support.TESTFN
 
 
 class TestSupport(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        orig_filter_len = len(warnings.filters)
+        cls._warnings_helper_token = support.ignore_deprecations_from(
+            "test.test_support", like=".*used in test_support.*"
+        )
+        cls._test_support_token = support.ignore_deprecations_from(
+            "test.test_support", like=".*You should NOT be seeing this.*"
+        )
+        assert len(warnings.filters) == orig_filter_len + 2
+
+    @classmethod
+    def tearDownClass(cls):
+        orig_filter_len = len(warnings.filters)
+        support.clear_ignored_deprecations(
+            cls._warnings_helper_token,
+            cls._test_support_token,
+        )
+        assert len(warnings.filters) == orig_filter_len - 2
+
+    def test_ignored_deprecations_are_silent(self):
+        """Test support.ignore_deprecations_from() silences warnings"""
+        with warnings.catch_warnings(record=True) as warning_objs:
+            _warn_about_deprecation()
+            warnings.warn("You should NOT be seeing this.", DeprecationWarning)
+            messages = [str(w.message) for w in warning_objs]
+        self.assertEqual(len(messages), 0, messages)
 
     def test_import_module(self):
         support.import_module("ftplib")
@@ -676,6 +705,17 @@ class TestSupport(unittest.TestCase):
     # SuppressCrashReport
 
 
+def _warn_about_deprecation():
+    # In 3.10+ this lives in test.support.warnings_helper
+    warnings.warn(
+        "This is used in test_support test to ensure"
+        " support.ignore_deprecations_from() works as expected."
+        " You should not be seeing this.",
+        DeprecationWarning,
+        stacklevel=0,
+    )
+
+
 def test_main():
     tests = [TestSupport]
     support.run_unittest(*tests)