pytest 8.2.2 breaks pytest-rerunfailures for tests that inherit unittest.TestCase

## pytest 8.2.2 breaks pytest-rerunfailures for tests that inherit unittest.TestCase

The code: (placed in `new_file.py`)

```python
from unittest import TestCase

class MyTestClass(TestCase):
    def test_base(self):
        self.fail()
```

Run command:

```bash
pytest new_file.py --reruns=1
```

Expected output: (Using `pytest==8.2.1` / previous version)

```
self = <examples.new_file.MyTestClass testMethod=test_base>

    def test_base(self):
>       self.fail()
E       AssertionError: None

new_file.py:5: AssertionError
```

Regression output: (Using `pytest==8.2.2` / latest version)

```
self = <TestCaseFunction test_base>

    def runtest(self) -> None:
        from _pytest.debugging import maybe_wrap_pytest_function_for_tracing
    
        testcase = self.instance
>       assert testcase is not None
E       AssertionError
```

The error occurs during the `rerun` of the test.

The failure line should be `self.fail()`. Instead, it's `assert testcase is not None`.
<b>This causes the `rerun` to never happen. (The test only ran the first time)</b>

Tested on a Mac. Here's the `pip list`:

```bash
Package              Version
-------------------- -------
iniconfig            2.0.0
packaging            24.0
pip                  24.0
pluggy               1.5.0
pytest               8.2.2
pytest-rerunfailures 14.0
```

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pytest 8.2.2 breaks pytest-rerunfailures for tests that inherit unittest.TestCase #12424