Allow multiple arguments to be passed to EmberStringUtils.fmt() Fixes #4383

This allows you to use `Ember.String.fmt` with the same multi-arg signature as `String#fmt`:
```js
Ember.String.fmt('%@ %@', 'John', 'Doe') // => "John Doe"
```

I don't think it's possible to also allow `String#fmt` to accept the array form of the current `Ember.String.fmt` as discussed in #4383, however. There's no way to tell whether the caller intends to use the `toString` value of the array as a format parameter or to use the array's values as format parameters.

I also noticed that most of the `tests/system/string` files were named incorrectly which was preventing them from being built and executed.