Add test for the integration with `solidus_backend`

[elia]

What

QA the backend integration, make sure all the necessary partials and behaviors are there and work as expected.
Add missing specs to cover any regular or edge case.

### Test cases
- [x] refunding a payment
- [x] voiding a payment
- [x] canceling an order
- [ ] https://github.com/solidusio/solidus_stripe/issues/266
- [x] navigating payments
- [x] inspecting/rendering log entries

Why

It's likely that once finished we'll still have some edge cases to cover, this is meant as a final QA that should be done before releasing.

[jakepenn]

Is there an integration with the payment element on a branch somewhere? I'm interested in using this in our shop at the moment

[elia]

@jakemumu yes, master is tracking that work, but it's still being heavily changed, especially where it maps models between stripe and solidus. Anyway one of the next PRs should put it into a relatively more stable state.