Review product option handle uniqueness and fix test

[glennjacobs]

The new product option editing on product variants allowed for duplicate product option handles. This is to allow for both shared (global) and exclusive product options.

This broke the following test https://github.com/lunarphp/lunar/blob/1.x/tests/core/Unit/Models/ProductOptionTest.php#L31-L47

We need to decide what we're doing to do with the handle field. E.g. is it of any use if it's not unique?