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
```
Merge pull request #5823 from ivanov/verbose-skip-js

be explicit about skipping js tests