be explicit about skipping js tests

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
```