gh-155207: Use the ExtraAssertions mixin in test_clinic

unittest.TestCase.assertStartsWith() and assertEndsWith() were added in
3.14.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_clinic.py

@@ -6,6 +6,7 @@
 from test import support, test_tools
 from test.support import os_helper
 from test.support.os_helper import TESTFN, unlink, rmtree
+from test.support.testcase import ExtraAssertions
 from textwrap import dedent
 from unittest import TestCase
 import difflib
@@ -2609,7 +2610,7 @@ def test_disallow_defining_class_at_module_level(self):
         self.expect_failure(block, err, lineno=2)
 
 
-class ClinicExternalTest(TestCase):
+class ClinicExternalTest(TestCase, ExtraAssertions):
     maxDiff = None
 
     def setUp(self):