[3.12] gh-71339: Add additional assertion methods in test.support (GH-128707) (GH-128815) (GH-129059)

Add a mix-in class ExtraAssertions containing the following methods:

* assertHasAttr() and assertNotHasAttr()
* assertIsSubclass() and assertNotIsSubclass()
* assertStartsWith() and assertNotStartsWith()
* assertEndsWith() and assertNotEndsWith()

(cherry picked from commit c6a566e47b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit 06cad77a5b)

Lib/test/test_venv.py

@@ -25,6 +25,7 @@ from test.support import (captured_stdout, captured_stderr,
                           requires_resource, copy_python_src_ignore)
 from test.support.os_helper import (can_symlink, EnvironmentVarGuard, rmtree,
                                     TESTFN, FakePath)
+from test.support.testcase import ExtraAssertions
 import unittest
 import venv
 from unittest.mock import patch, Mock
@@ -58,7 +59,7 @@ def check_output(cmd, encoding=None):
             p.returncode, cmd, out, err)
     return out, err
 
-class BaseTest(unittest.TestCase):
+class BaseTest(unittest.TestCase, ExtraAssertions):
     """Base class for venv tests."""
     maxDiff = 80 * 50
 
@@ -98,10 +99,6 @@ class BaseTest(unittest.TestCase):
             result = f.read()
         return result
 
-    def assertEndsWith(self, string, tail):
-        if not string.endswith(tail):
-            self.fail(f"String {string!r} does not end with {tail!r}")
-
 class BasicTest(BaseTest):
     """Test venv module functionality."""