Added unit tests to widget Breadcrumbs and refactor of Breadcrumbs::renderItem()

Yii 2 framework do not use:

```php
if ($expression) {
    return ...;
}

return ...;
```

Original code is fine:

```php
if ($expression) {
    return ...;
} else {
    return ...;
}
```

You can check other sources for ensure.