unittest.TestCase is not multiple-inheritance cooperative

Bug report

Bug description:

unittest's TestCase does not call super().__init__() which breaks cooperative multiple inheritance.

I would be happy to add this, but I can imagine someone somewhere relies on cooperative multiple inheritance not working.

CPython versions tested on:

3.11, 3.12, 3.13, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-125287

从 unittest.TestCase 的初始化开始，并在 CPython 上复现协作式多重继承的情况。检查其在所报告的 Python 版本中的行为，并确定相关的 unittest 测试。完成的标准是：TestCase 能够与多重继承协作而不破坏现有的初始化行为，并有一个覆盖该情况的回归测试。