Improve test coverage on shipping methods

[filipefurtad0]

What we should change and why (this is tech debt)

We currently have few tests around the shipping method feature, making this a vulnerable point in the app.

Context

This came up while chatting on Slack with @luisramos0 (via PM). Also, several issues related to this were recently found while testing #6039 :

• Can't delete shipping methods if these were used in previous orders #6135

• Setting all available shipping methods as "backoffice only" hides all of them from the backoffice as well  #6136

• Checkout still possible despite all shipping methods set to "backoffice only" #6137

Impact and timeline

This would give us more confidence in future upgrades, as well as reducing the need to test this feature manually.