Wait until all listeners are removed before finishing test

Without this change, running just the test/add_test.js tests, Node never exits.  E.g.

    ./node_modules/grunt-contrib-nodeunit/node_modules/.bin/nodeunit test/add_test.js

I'm trying to test some changes and having trouble getting the tests to run properly.  I'm guessing there may be other similar problems, but this one was the clearest.