Upgrading VTU to v1: Remove deprecated `methods` usage in alerts_service_form_spec

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":

• unify nextTick as per gitlab-org/frontend/rfcs#47 (closed)