Some gardening on iptest result reporting
Fix display of notebook directory path
Refactor TestController API to allow it to display extra info.

Add a setup() method to be called when we know we're going to use a test
group, for creating temporary dirs etc., and a print_extra_info() method
to display extra information.
Refactor generation of casperjs command
Shorten leftover references to 'IPython test group'
Improve description of -j option to iptest
Merge pull request #5326 from takluyver/iptest-gardening

Some gardening on iptest result reporting