unittest: add addDuration function for Python 3.12 support

Fix pytest-dev#10875

Without this, fails with

```
...
E           AttributeError: 'TestCaseFunction' object has no attribute 'addDuration'
...
E           RuntimeWarning: TestResult has no addDuration method
```

src/_pytest/unittest.py

@@ -298,6 +298,9 @@ def addSuccess(self, testcase: "unittest.TestCase") -> None:
     def stopTest(self, testcase: "unittest.TestCase") -> None:
         pass
 
+    def addDuration(self, testcase: "unittest.TestCase", elapsed: float) -> None:
+        pass
+
     def runtest(self) -> None:
         from _pytest.debugging import maybe_wrap_pytest_function_for_tracing