Improve testing for mediator pattern

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The mediator pattern poses some issues when testing actions, as the testAction helper we use to test our actions doesn't use an actual Vuex store and thus doesn't know about the mediator at all, which makes it hard to fully test the flow for mutations that trigger other actions/mutations from the mediator.

We can still write specs for the mediator itself but doing this decouples the logic from the original action, which can lead to some regressions while refactoring code.

/cc @samdbeckham