gh-106300: Improve assertRaises(Exception) usage in test_runner

[sobolevn]

Explaining changes:

1. I've refactored all raise Exception to raise our new CustomError, so we can use it with self.assertRaises(CustomError)
2. To fix string-based tests I've added CustomErrorRepr helper, so it would work for two styles of test execution (direct and via regtest):

(.venv) ~/Desktop/cpython  main ✗                                                         
» ./python.exe -m test test_unittest.test_runner    
0:00:00 load avg: 1.21 Run tests sequentially
0:00:00 load avg: 1.21 [1/1] test_unittest.test_runner

== Tests result: SUCCESS ==

1 test OK.

Total duration: 253 ms
Tests result: SUCCESS
                                                                                           
(.venv) ~/Desktop/cpython  main ✗                                                         
» ./python.exe Lib/test/test_unittest/test_runner.py
..................................................
----------------------------------------------------------------------
Ran 50 tests in 0.225s

OK

3. I've fixed the bug from assertRaises(Regex)?(Exception) is problematic #106300 (comment) Now TestableTest.doClassCleanups() is not expected to raise any exceptions :)

• Issue: assertRaises(Regex)?(Exception) is problematic #106300

[serhiy-storchaka]

On one hand, most of these changes are not necessary, because exceptions are differentiated by their messages.

On other hand, a bug similar to the bug in doClassCleanups() testing may occur again, and a custom exception will help catch it.

LGTM. Thank you.

[miss-islington]

Thanks @sobolevn for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-108006 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-108007 is a backport of this pull request to the 3.11 branch.