Added link to qunit-once plugin

[qunit-once](https://github.com/bahmutov/qunit-once) makes it easy to run a method once at the start of a module, and once after. After including qunit-once you get two additional functions

```js
QUnit.module('Example', {
  setupOnce: function () {
    // runs once before anything else in the module
  },
  setup: function () {
    // runs before each unit test
  },
  teardown: function () {
    // runs after EACH unit test in this module
  },
  teardownOnce: function () {
    // runs once after all unit tests finished (including teardown)
  }
});
```