Added unit tests for Widget\ActiveField

unit test from widget\ActiveField Class.
if you note, my $content is being wrapped with the begin tag ``<div class="form-group field-dynamicmodel-attributename">``
and with the end tag.
my wrapper;
```php
$content = function($field) {
 return "<div class=\"custom-container\"> $field </div>";
};
```
my expected output:
```html
<div class="custom-container"> <div class="form-group field-dynamicmodel-attributename">
<label class="control-label" for="dynamicmodel-attributename">Attribute Name</label>
<input type="text" id="dynamicmodel-attributename" class="form-control" name="DynamicModel[{$this->attributeName}]">
<div class="help-block"></div>
</div> </div>
```
but the actual output wraps my oun wrapper with the begin and end tag with attributes from the element being rendered.
```html
<div class="form-group field-dynamicmodel-attributename"><!-- HERE -->
<div class="custom-container"> <div class="form-group field-dynamicmodel-attributename">
<label class="control-label" for="dynamicmodel-attributename">Attribute Name</label>
<input type="text" id="dynamicmodel-attributename" class="form-control" name="DynamicModel[{$this->attributeName}]">
<div class="help-block"></div>
</div> </div>
</div><!-- HERE AGAIN -->
```
is this the expected output? i'm might be missunderstanding the ``render()`` with a ``$content``.