Created tests for and fixed bug in get/setSelectedModels by offset for table view

Changing selection with arrow keys in the table view changing wasn't working because of a problem with:
```
get/setSelectedModel/s({ by: "offset" })
```
It was using:
```
this.$el.find( "> [data-item-id]:visible" )
```
to get the item elements, which worked fine for the ```ul``` views, but not for ```table``` views. I added a conditional that checks if the view is rendered as a ```table``` and if so, uses this selector instead:
```
this.$el.find( "> tbody > [data-item-id]:visible" );
```