Make Vuex action test helper fail if more actions are dispatched / mutations committed than expected

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18072#note_65793631:

The Vuex action test helper checks

• mutationsCount >= expectedMutations.length
• actionsCount >= expectedActions.length

We should fail the tests if mutationsCount > expectedMutations.length or actionsCount > expectedActions.length.