Upgrading VTU to v1: Remove deprecated `methods` from components/checkout (!50498) · Merge requests · GitLab.org / GitLab · GitLab

What does this MR do

This MR is part of &3856 (closed) and &5102

This change switches from using deprecated methods option in mount / shallowMount to action spies on Vuex

Additional changes "while we are here":

remove localVue usage as per #220482 (closed)
unify nextTick as per gitlab-org/frontend/rfcs#47 (closed)
switch from then to async / await as per gitlab-org/frontend/rfcs#45 (closed)