'Run the "test_func" Item.
The calling test instance (the class which contains the test
method) must provide a ``.getrunner()`` method which should
return a runner which can run the test protocol for a single
item, like e.g. :py:func:`_pytest.runner.runtestprotocol`.'

def runitem(self, source):

    item = self.getitem(source)
    testclassinstance = self.request.instance
    runner = testclassinstance.getrunner()
    return runner(item)
