Code review comment for lp:~spiv/bzr/traceback-accumulation-2.2

As the comment in the patch says this is something I think testtools ought to provide, but it seems to me that as a practical matter it's easiest to include a fix in bzrlib that works even with older, unfixed testtools. We often take the same approach with other dependencies, like Python itself. I realise that testtools takes patches and releases new versions more frequently and readily than Python, but it's considerably more effort to get all of PQM, babune, and developers' systems updated with a new testtools than it is to just put a fix in bzr now, and allow those to be updated at leisure.