Adding tail node tests

Adding unit tests for the tail node. Had to add a Node test <code>helper.js</code> <code>stopServer(done)</code> to stop the tail nodes littering the test output with warnings when the tailed file gets deleted in the <code>afterEach(done)</code> calls. I know it's not the most efficient that the server instance gets re-initialized for each test but improvements can be considered later if we find it's an issue. Thanks