Update JSON file access by karma/jasmine tests

Existing node JS tests (in `js/test`) use a combination of gulp and phantomjs to run.

phantomjs is now unsupported (and seems to be a pain to get working locally) so it would be good to refactor the tests to use karma/jasmine.

I've had a quick go at this but got stuck trying to get the test functions to access the CSV files. (If we have to we could convert them to JSON as part of the test script.)