[noble+] TestResult has no addDuration method with Python 3.12

[Impact]
Spurious warnings in tests since Python 3.12, might cause tests to fail when asserting that stderr is empty.

[Test plan]
Run the update-manager autopkgtest and see no such warning.

[Where problems could occur]
There is no regression potential basically. The code creating the warning in Python 3.12+ looks like:

    def _addDuration(self, result, elapsed):
        try:
            addDuration = result.addDuration
        except AttributeError:
            warnings.warn("TestResult has no addDuration method",
                          RuntimeWarning, 1)
        else:
            addDuration(self, elapsed)

that is, if the addDuration method does not exist, it logs a warning and does nothing else. By adding the addDuration method, the warning is not printed.

The addDuration method adds the execution time of the test to the test result, but nose's test results does not track that, so meh.