Allow for plugin testing.

With this change in place you can include plugin specs in `grunt test` by creating a `src/config.js` like that includes something like this:

```js
      plugins: {
        // list of plugin panels
        panels: [],
        // requirejs modules in plugins folder that should be loaded
        // for example custom datasources
        dependencies: ['grafana-plugins/atlasDatasource'],
        specs: ['grafana-plugins/specs/atlasDatasource-specs']
      }
```