Message 181800 - Python tracker

My concern is that this re-uses the existing TestResult.add* methods in a different way (including calling addError multiple times). This can break existing tools.

Fix suggested by lifeless on IRC. A sub test failure / success / exception calls the following TestResult method:

   addSubTest(test, sub_id, err_or_None)


If we're using a TestResult object that doesn't have these methods (an "old" result objects) then the test can *stop* (i.e. revert to the old behaviour before sub tests existed).