Docs:

tearDown()

    Method called immediately after the test method has been called and the result recorded. This is called even if the test method raised an exception, so the implementation in subclasses may need to be particularly careful about checking internal state. Any exception, other than [AssertionError](https://docs.python.org/3/library/exceptions.html#AssertionError) or [SkipTest](https://docs.python.org/3/library/unittest.html#unittest.SkipTest), raised by this method will be considered an additional error rather than a test failure (thus increasing the total number of reported errors). This method will only be called if the [setUp()](https://docs.python.org/3/library/unittest.html#unittest.TestCase.setUp) succeeds, regardless of the outcome of the test method. The default implementation does nothing.

Some parts that are important:

This is called even if the test method raised an exception, so the implementation in subclasses may need to be particularly careful about checking internal state.

In this case test method is not called. But, we have a guard in the docs. Callers must be careful with its internal state.

This method will only be called if the setUp() succeeds, regardless of the outcome of the test method.

This gives me confidence in this patch. It does what the docs say. If setUp() is successful, we need to run tearDown().

Things I like about the implementation:

• It is simple
• It does not polute TestCase, only IsolatedAsyncioTestCase
• But, it still gives some flexibility to TestCase subclasses: _callCleanupAfterFailedSetup can be adjusted in user-defined classes if needed

Do we need to better document it? I would prefer to hide it for now for end users.

• Issue: unittest: tearDown is not called if asyncSetUp fails #96139