Run all individual river tests when running npm test

[rhyolight]

Currently to test a river, you run:

node test-river.js <river-name>

and the test script will find the river config and parser and run tests against it. I would really like this test to run against all rivers in the rivers directory every time someone runs npm test. It would be nice to be able to test one river from the command line as well, but not essential.