add an option to disable the warning for tests with no assertions


I did not find any information how to test the gruntfile itself, so here is a proof of the working status:

```
>grunt qunit:warnings_disabled
Running "qunit:warnings_disabled" (qunit) task
>> 0 assertions passed (0ms)

Done, without errors.

>grunt qunit:warnings_enabled
Running "qunit:warnings_enabled" (qunit) task
Warning: 0/0 assertions ran (0ms) Use --force to continue.

Aborted due to warnings.
```

I'm open to shorter names for this option :) Maybe we could name it "strict" and apply 
other good practices for testing here (see strict mode from phpunit)

I hope you did not mind removing some trailing commas here and there...

best regards
philipp