improve node-fast artedi tests

[joshwilsdon]

With #9 and #17 node-fast has basic artedi support, but the tests are not exhaustive. @davepacheco suggested we have additional tests that validate the numbers returned but doing a good job of this will be blocked on node-artedi being able to expose these metrics in a format conducive to testing. Something like TritonDataCenter/node-artedi#11 could work. Alternatively artedi could allow a artedi.FMT_JSON format for collector.collect(). However this is resolved, once it is we should go and add more extensive tests to node-fast for the artedi support.