Improve Vuex test

[afontcu]

👋

When started working with VTL, I remember Vuex was one of the pain points when testing my components.

So in this PR I did a couple of things:

1. I created "a store" by moving the object to an external file and importing it (as a developer would in a real project). For that I created a /Store folder to match the Router tests structure.
2. I used the render wrapped pattern to showcase how to create isolated tests. I also wrote about it, so people can understand the reasoning behind the code. In the current implementation of Vuex testing examples, swapping the first two tests makes one of them fail (because of shared state).

As usual, this PR is just a proposal. I'm totally open to improvements and tweaks :)