Some gardening on iptest result reporting

My nice concise output  when running `iptest -j6` was being messed up by some lines like "Running js/tree tests in directory: '/tmp/tmp994_q5'" that had snuck in. There wasn't a good way for the test process to add potentially useful information on failure without it showing up in the concise view on success. This adds that: a `TestController.dump_failure()` method which prints stdout from the test process. `JSController` overrides this to add the notebook directory.

While looking at this, @ivanov pointed out that "IPython test group" was unnecessary, so I shortened it to "Test group".