If the test controller tries to read the file while the server it has
started is in the middle of writing it, it gets invalid JSON and fails.
This just loops again until we have valid JSON to read.
this way those running the test suite can discover that there are a
bunch of js test groups. New output of `iptest -j` looks like this
```
Test group: kernel.inprocess -------------------------------------- OK
Test group: config ------------------------------------------------ OK
Test group: extensions -------------------------------------------- OK
Test group: lib --------------------------------------------------- OK
Test group: testing ----------------------------------------------- OK
Test group: core -------------------------------------------------- OK
Test group: terminal ---------------------------------------------- OK
Test group: utils ------------------------------------------------- OK
Test group: kernel ------------------------------------------------ OK
Test group: nbformat ---------------------------------------------- OK
Test group: qt ---------------------------------------------------- OK
Test group: autoreload -------------------------------------------- OK
Test group: html -------------------------------------------------- OK
Test group: nbconvert --------------------------------------------- OK
Test group: parallel ----------------------------------------- NOT RUN
Test group: js/base ------------------------------------------ NOT RUN
Test group: js/notebook -------------------------------------- NOT RUN
Test group: js/widgets --------------------------------------- NOT RUN
Test group: js/tree ------------------------------------------ NOT RUN
Test group: js/casperjs -------------------------------------- NOT RUN
Test group: js/services -------------------------------------- NOT RUN
```
- set linger on sockets (avoid hang on exit)
- actually close sockets, not just zmq streams (blame pyzmq, aka me)
seems to be necessary for js/notebook/shutdown test to avoid hanging on Python 3.4.