Code review comment for lp:~msabramo/capturemock/tests_add_mutable_mapping

Thanks for the test, I've now merged it in. The main change I've made, as I've not been able to fix this yet, is to add a "known bug" in TextTest, telling it that the exception text on standard error implies the presence of the linked bug, so that the test does not disturb others working with the tests and doing other stuff. (I also reordered the tests, I try to keep simple tests near the top and more complex ones near the bottom)