def runitem(self, source: str) -> Any:
        """Run the "test_func" Item.

        The calling test instance (class containing the test method) must
        provide a ``.getrunner()`` method which should return a runner which
        can run the test protocol for a single item, e.g.
        ``_pytest.runner.runtestprotocol``.
        """
        # used from runner functional tests
        item = self.getitem(source)
        # the test class where we are called from wants to provide the runner
        testclassinstance = self._request.instance
        runner = testclassinstance.getrunner()
        return runner(item)
Run the "test_func" Item.

        The calling test instance (class containing the test method) must
        provide a ``.getrunner()`` method which should return a runner which
        can run the test protocol for a single item, e.g.
        ``_pytest.runner.runtestprotocol``.
        
src/_pytest/pytester.py